It must be factual, not based in speculation.
The primary purpose of Skeptics is to verify or falsify claims. If an answer an be summed to "I don't know for sure, but here is what I think the answer is," then it is a bad answer as it unhelpful - it neither verifyverifies nor falsifyfalsifies anything.
Speculation is not completely bad. For example, an answer that shows, without speculation, that a claim is false, and then provides some speculation as to how the claim might have sprung from a misunderstanding is certainly a good answer.