Shouldn't the alternative correct solution given (see above) be 'fuest' and not 'buest' since this is a question?
There is not mutation after 'Pryd' so the two alternatives are 'Pryd buest ti 'na?' and 'Pryd buoch chi 'na?'
So without 'pryd', 'buest' would become 'fuest' in any other question form? For example, 'fuest ti yn y digwyddiad?'
Not in every other question form - there is no mutation in the usual colloquial language after:
Diolch am eich cymorth