It isn't. Plaid Cymru is nowadays usually known as 'Plaid Cymru' in English as well as in Welsh.
As explained above, and in the notes for this part of the course, Plaid Cymru is not usually translated into English. The party calls itself 'Plaid Cymru' in both languages.
Since Plaid Cymru does have an English variant, although that's not used very often, that is now included as the main translation for this exercise. Thanks for the comment.
Given that Llywodraeth Cymru is translated as 'The Welsh Government', why is 'The Welsh Party' not accepted as a rendering of Plaid Cymru? (I appreciate that the name is not normally translated st all, but it seemed like an evasion merely to copy the Welsh words into the box.)
This is all about how we translate possession between the languages. This video explains it well:-
Although the slight variation here is the lack of the second 'the' in the English, but it's the same rule.
Thus:- 'The Party of Wales' (which is the official English title of Plaid Cymru) becomes 'Party Wales' = Plaid Cymru
Thus 'The Welsh Government' = 'The Government of Wales' - cross out 'The' and 'of' = 'Government Wales' = 'Llywodraeth Cymru'
The reason for not calling Plaid Cymru 'The Welsh Party' and preferring 'The Party of Wales' is no doubt because there is more than one political party in Wales and so it would be difficult to differentiate in English between Plaid Cymru and eg 'The Welsh Labour Party'; 'The Welsh Green Party' etc.
Plaid Cymru was originally known as 'Plaid Genedlaethol Cymru' (The National Party of Wales).