Translation:They want to drink beer on Thursday.
From what I have read, this is a dialect distinction - north Wales would tend to have the mutation here while south Wales traditionally does not mutate "dydd" in such situations, and that 'quirk' has such a long tradition there that it cannot be called wrong.
So for a course that accepts all variants of Welsh, both should be accepted and marked correct.
Which variant is presented to students is a matter for debate. A case could probably be made for "show all variants at different times, so that learners get accustomed to reading material in other dialects that may obey different grammatical rules from the one the user is focussing on for themselves", as well as one for "show only one main dialect so that what students will acquire will be more natural (if biassed towards one region) rather than a sort of composite, non-regional dialect that is nobody's native one".