Why does the nominalization of the verb trả lời requires the classifier câu to become a determined noun the answer, but đáp án not, or, if ever, not with câu but with cái? For me both look exactly the same: both are compound nouns and go together as a whole. Or is this an exception?
trả lời is a verb, you need "câu" to nominalize it.
đáp án can be both a verb or a noun. it needs nothing to be nominalized. beside, a lot of compound words, mostly Sino-Vietnamese words, don't usually take classifiers.
here's a link to a list of nouns that don't take or at least don't need classifiers.