The MEANING are different in "which is/are the answer?" and "what is the answer?".
On the first one, it is assumed that there are alternatives and you are asking which one of them is the answer.
On the second one you are objectively asking WHAT IS the answer.
In portuguese, when we say "Qual é a resposta", the meaning is exactly like "What is the answer". If we wanted to say "Which is the answer?" we would've said "Qual dessas é a resposta?".
Trust me, I'm a native speaker of portuguese! :)
Did not accept "What is the reply". The word "answer" is more general than the word "reply" (in english) in that an answer can be any general "solution" to a problem but a "reply" is specifically a response to a question. So I have to take this to mean that "resposta" in Portugese means a general solution and not a reply. And then there is the word "Responda"... realy confusing.