Scrum Role

The question is a bit jumbled. It should probably be worded like this:

Q: When multiple teams are working on the same product, one person can be member of more than one Scrum team.

A: True

The answer is True. The explanation is not the best. There are no best practices when it comes to solving complex problems, team structure, etc. When taking the PSM remember that the Scrum Guide is intentionally incomplete, a framework (not a methodology), and is not a "how to" guide. And, the Scrum Guide mentions accountabilities, not roles.

I feel this part of the explanation is good:

The Scrum guide does not prohibit it

Scrum would not forbid a member to be part of more than one Scrum team. Your theory is good, as focus could be lost in many situations, but there could be some situations where it works just fine. If focus is being lost, hopefully the issue could be made transparent and the members of the teams can inspect and adapt to find a better solution.

All the best with the PSM.

PS - Beware of external Scrum exams that are not part of this web site, many of them will push you in the wrong direction.

A person can be member of of scrum team A, scrum team B, Scrum Team C etc. How he/she stay focus on the task list from respective scrum team during the sprint?

Think of it the other way round. Would adequate focus be so implausible with such an arrangement that there is reason to prohibit it?

In order for the answer to be true, you'd only need one example where it's OK for a single person to play a role on multiple Scrum Teams. The Scrum Guide gives this one explicit example:

If Scrum Teams become too large, they should consider reorganizing into multiple cohesive Scrum Teams, each focused on the same product. Therefore, they should share the same Product Goal, Product Backlog, and Product Owner.

The Product Owner is a member of the Scrum Team. In the case of multiple Scrum Teams working on the same product, all of the teams share the same Product Owner. Therefore, the answer to this question must be "true".

However, I don't think that the Product Owner is the only one that this applies to. I'd suggest that it's good for the Scrum Master to play the part on multiple teams in this situation. Since the Scrum Master is facilitating the different events and promoting good, positive interactions, playing the role on multiple teams makes it easier to facilitate the events that affect multiple teams and help the interactions between the teams.

I'm not convinced that a Developer should be on more than one Scrum Team. This would seem to prevent focus on a singular Sprint Goal and Sprint Backlog, assuming that each team has their own Sprint Goal and Sprint Backlog. However, there may indeed be some cases where the Developer is really a specialist offering their services to all of the teams, so I don't think you can say that Developers should never be on more than one Scrum Team.