Using は is grammatically correct, but it sounds like you’re generally stating the fact and about to start explanation. If you want to pick one among multiple choices, you’d usually add が to emphasizes on the answer. To make it more naturally, we’d say 二番の答えが正しいと思います。Hope it makes sense.
In Japanese, you can say “This answer is mistake.” この解答は誤りです。Yet, it sounds very かたい and it’s something you’ll find in a textbook. I can’t think of any other examples, but when you want to casually say something is wrong or incorrect, you need to take the て form: まちがっている (or まちがっていた in past tense). Just as a note, this い can be omitted in conversations, so some people just say まちがってる or まちがってた.