Is there a term for a question and answer pair?

I'd call it a...

question and answer interchange (21,300 hits on Google)

Or just plain...

interchange - mutual interaction; the activity of reciprocating or exchanging (esp. information)

Supplementing FF's answer:

Or just:

Question and answer
Q & A

This might be esoteric and on the edge, but we don't really know the context for OP's query:

What came to mind was a "challenge". When you can't remember your password on your banking site, as an example, they will give you a "challenge", a "challenge question", or a "security question" (I've heard all three). They would seem different from a regular question in that there is supposed to be only one acceptable answer paired to the question portion of the challenge. A soldier on guard duty might also issue a challenge that would need to be returned with the correct paired answer.

Just an idea. "Challenge" was single word as requested. Like I say, we don't have a lot of information.

This sounds like a request for a computer program variable name.

In that domain, related pairs of things are often referred to as "tuple"s or "pair"s. So if it were me, I'd name that variable question_answer_pair, or perhaps qa_pair for short.