What part of the capacity of the Development Team does Product Backlog refinement usually consume?

Which mock test were you using?

In previous versions of the Scrum Guide, there was a recommendation that teams reserve about 10% of their capacity for Product Backlog Refinement. However, this was removed from more recent versions. If the mock test is stating this recommendation as a fact, not only is it incorrect as it was never a requirement, but it also calls into question every other question since it doesn't reflect the most recent thinking about Scrum as its defined in the Scrum Guide.

Enough to have a Product Backlog with enough items to be pulled into a Sprint that are fully understood by the entire Scrum Team and stakeholders. 

I echo @Thomas' sentiments about the assessment.  There are a lot of free and paid resources on the internet that cause more harm than good when trying to understand the Scrum framework. Study the official Scrum Guide that is free for all. It provides the current information on the Scrum framework. It is revised periodically.  Resources from other places can not be guaranteed to be updated at all.

Strictly speaking the Developers are not "authorized" for Product Backlog refinement, because no authorization is needed. They'd do it anyway, and they would refine it enough. Scrum isn't about authority, it's about accountability.

So the last answer is arguably the least worst...but it is abundantly clear that the framers of the question do not understand the present Scrum Guide, and hence you can have little confidence in the test they have produced.

thank