Would "Khi tôi hỏi, bạn trả lời" also be acceptable?
Yes, it would be.
I just gave that answer, and it was accepted.
what is the difference between trả lời and đáp án?
"Tra loi" is a verb, while "dap an" is a noun.
You may use "tra loi" as a noun by adding "cau"(which means sentence) in front.
So, I guess "cau tra loi" and "dap an" are basically the same.
I hope it helps, and please correct me if I'm wrong.
When I ask, THEN you answer. If we're going to learn this, the lessons need to be consistent. AND they need to be explained.