I've seen the question before and, I believe, the answer they're after is 'b'. If its any consolation I think the wording is more definite in the question bank used by the CAA.
Something like: What is the purpose of a request from an ATSU to 'verify your FL'?
a) Check the accuracy of the Mode C readout displayed to the Controller,
b) Confirm accuracy of altimeter.
If you think that this question is ambiguous then you'll enjoy the HP and L exam...
Good Luck