Translation:A capital city
Prifddinas comes from prif ("chief", borrowed from Latin prīmus, "first") and dinas ("city", augmentative of din ["fortress"], and related to English down ["hill; field; tract of sandy land near the sea"]).
Why isn't "capital" accepted? "Cardiff is the capital of Wales" is surely good English.
The word 'capital' on its own could be several other things that are nothing to do with 'capital city' - a capital letter, the fancy top of a column, a sum of money, ...
It isn't. Both "capital city" and "a capital city" are (or should be) acceptable responses.