would "tôi muốn một đáp án" be equivalent to this?
yes it should be accepted.
In that case, would đáp be the classifier for đáp án? or would câu work here? or does it just not use a classifier?
"đáp án" goes together as a whole so there is no classifier. In very informal case, "cái" can be used as classifier for "đáp án".
Why is "I want the answer" wrong in this case?
If it has the word một (one) it always translates with "a" rather than "the"
This might be a dumb question but what does "câu" do to this sentence? I either forgot or missed something on mobile.
it nominalizes the verb trả lời (to answer) into a noun (an answer). many consider it a classifier denoting sentences, expressions, lines.
I thought that "trả lời" was the VERB form of "answer" and "đáp án" was the NOUN. In this sentence, "answer" is a noun, so should be "đáp án".