Development team composition and PO typical activities

A Development Team should self-organize in order to meet the Sprint Goal. If that means changing team composition mid-sprint then so be it, but it would be a drastic action and the repercussions could be more disruptive than beneficial.

A new Sprint commences immediately upon the termination of its predecessor. The only activity that any team members typically engage in between a Sprint Review and Sprint Planning is the Sprint Retrospective. There is no time for anything else.

About the second quesiton: 2. Are there any typical activities that a Product Owner would undertake in the phase between current sprint review and the next sprint planning? updating project plan with stakeholders, refining product backlog, or there are no such activities.

The answer here should be Sprint Retrospective but there is no option for it. Then the most suitable could be No such activities?

The Sprint Retrospective and review happen at the end of a sprint. When a sprint ends, a new one is immediately started.

A sprint starts with a planning session. The retrospective happens at the end of a sprint so that the entire sprint can be inspected upon and adapted.

Updating stakeholders on the project plan, refining the product backlog should be done continuously and iteratively. Posting the release burndown in a visible and accessible location and keeping it updated can be one way of emitting project plans to stakeholders.

I could fully agree with you, if we talked about Core-Scrum.
But the Scrum Guide does not say, that a Sprint has to begin with the Planning meeting. Core Scrum does!
The Guide only tells us, that the work for the Sprint is planned in the Sprint planning.
So if a review shows, that the backlog has to be changed significantly one could do a Backlog refinement session together with the team before the planning.
From this point of view, the answer could possibly also be: "Refining the Backlog".
I am not sure if "there is no time for anything else".

The Scrum Guide says, with regard to Sprint Planning:

"Work planned for the first days of the Sprint by the Development Team is decomposed by the end of this meeting, often to units of one day or less". No exception is made, at the beginning of the Sprint, for anything to occur other than planning for this immediate start.

It's quite acceptable for outstanding Product Backlog refinement to be taken care of in the Review of the *previous* Sprint. This includes assessing undone work and revising the Product Backlog accordingly. However, it wouldn't be acceptable to roll the debt from one time-box (such as inadequate refinement) over into the next one, and to try and deal with it before planning commences. To do so would amount to robbing the next Sprint of the time available.

What activities would a Product Owner (PO) typically undertake in the phase between the current Sprint Review meeting and the next Sprint’s Planning

The correct answer should be A: "Refining the Product BackLog".

We are currently in the Sprint Review meeting. That means that at the end of it the "Result is a revised PB defining the probable PBI for next Sprint". After this meeting the PO should revised (detailing contents, check the order etc.) the PB in preparation to the upcoming Sprint Planning.

The Answer D "There are no such activities. The next Sprints starts immediately after the current sprint" cannot be correct. On one hand - this would mean that the PO can stop doing anything (he/she can go golfing) until the next Sprint and on the other hand we all know there is an activity inbetween called "Sprint Retrospectiv", where the PO has to be present. This answer would be correct, if we were in the Sprint Retrospective - but war are not.

And by the way "...the next sprint starts after the current sprint". What a statement! After today comes tomorrow - is a tautological scentence.

The correct answer should be A: "Refining the Product BackLog".

The Scrum Guide describes refinement in the section where the Product Backlog is discussed. It says 

Product Backlog refinement is the act of adding detail, estimates, and order to items in the Product Backlog. This is an ongoing process in which the Product Owner and the Development Team collaborate on the details of Product Backlog items. During Product Backlog refinement, items are reviewed and revised. The Scrum Team decides how and when refinement is done. Refinement usually consumes no more than 10% of the capacity of the Development Team. However, Product Backlog items can be updated at any time by the Product Owner or at the Product Owner’s discretion.

So why would the Product Owner be the only refining the backlog and only during the "break" between Sprints? This is an ongoing activity done by all in order to add clarity, understanding and agreement on the Product Backlog Items.

The Answer D "There are no such activities. The next Sprints starts immediately after the current sprint" cannot be correct. 

From opening paragraph from the Scrum Guide section that describes the Sprint (emphasis by me) 

The heart of Scrum is a Sprint, a time-box of one month or less during which a "Done", useable, and potentially releasable product Increment is created. Sprints have consistent durations throughout a development effort. A new Sprint starts immediately after the conclusion of the previous Sprint.

That is a tautological statement but it is specifically called out because people try to add a period of time between the Sprints to acccount for refinement, research and preparation for the next Sprint.  Those activities should be done within the Sprint as they are all necessary in order for the Scrum team to operate.

...and on the other hand we all know there is an activity inbetween called "Sprint Retrospectiv", ...

Another passage from the Scrum Guide, this one from the section that describes the Scrum Events (emphasis by me)

Other than the Sprint itself, which is a container for all other events, each event in Scrum is a formal opportunity to inspect and adapt something. These events are specifically designed to enable critical transparency and inspection. Failure to include any of these events results in reduced transparency and is a lost opportunity to inspect and adapt.

The Retrospective is a Scrum Event which is part of a Sprint and not something that is done between Sprints. I have always viewed the Sprint Retrospective as the end of a Sprint while Sprint Planning is the beginning. There is nothing in the Scrum Guide that explicitly states this but it is something that helps me understand the purpose of the events. 

@Daniel

So why would the Product Owner be the only refining the backlog and only during the "break" between Sprints?

You kindly confirmed with the quote "During Product Backlog refinement, items are reviewed and revised" you kindly confirmed that on the Sprint Review a Product Backlog refinement happens (which supports answer A). Product refinement is an ongoing process, but the informal Sprint Review meeting, is regarded as a good opportunity enabling critical transparency and inspection. Furthermore such "events are used in Scrum to create regularity and to minimize the need for meetings not defined in Scrum."

A new Sprint starts immediately after the conclusion of the previous Sprint.

We need to consider that a "Sprint" does "contain and consist of the Sprint Planning, Daily Scrums, the development work, the Sprint Review, and the Sprint Retrospective." and the question refers to the third event - the Sprint Review; "Although implementing only parts of Scrum is possible, the result is not Scrum. Scrum exists only in its entirety." Sprint Review and the Sprint Retrospective must take place before we start again.

"The Retrospective is a Scrum Event which is … not something that is done between Sprints."

If we look closely at the question, it does not refer to something between the Sprints. The question refers to the period between current Sprint Review and the next Sprint's Planning. The Sprint Retrospective does not really matter here, because an activity is already taking place beforehand. The answer is given by the Scrum Guide: "The result of the Sprint Review is a revised Product Backlog that defines the probable Product Backlog items for the next Sprint." - Such a result can be only obtained/determined through the activities from the PO/ Scrum Team respectively is an activity itself marks the end of the Sprint Review.

Please note that I only state my interpretation of the question/ scrum guide. To be 100 % sure, you need to contact the author of the question. All statements without guarantee.

@Flavio Ianniello  I appreciate your feedback and your opinion.  All any of us can do is provide our interpretion and opinions.  It is up to the people reading to decide how our input affects their interpretations and opinions. 

Thanks for your involvement.  I will admit that what you have said does raise a few questions for me which I find valuable.