Do Jeopardy! answers have to use the correct interrogative form?

In fact, they don't even require the standard "Who is", "What is", "Where is" construction. As long as the answer is in the form of a question, it is considered acceptable. You see this most often when the answer is a question by default, like "Who's Afraid of Virginia Woolf?" or "Where Have All the Flowers Gone?" If a contestant just gives the title without a "What is" tag, Alex Trebeck will usually make a comment like, "Our judges are accepting that, as it is indeed in the form of a question."

There was also a Celebrity Jeopardy contestant once who habitually answered questions using the construction: "The Emancipation Proclamation, what's that?" These answers were all accepted, though it seems unlikely the judges would be as tolerant with their ordinary contestants.

Unfortunately, I'm relying entirely on memory here, so I can't direct you to any actual show footage to back this up. But I've been watching the show regularly since the 80s, and I've definitely seen examples of this occurring periodically.

After watching just a few episodes, I've noticed that the correct interrogative form is not required for the answer to be considered correct.

For example, I would be shocked (along with most viewers) if Alex Trebeck replied to that answer with, "No, I'm sorry, that is incorrect." And another player gave the answer "Who is Christopher Columbus?", and Alex allowed that answer.