*Ответь should be changed to the imperative, whether in the second person or the plural, such as:
Ответьте на вопрос!
отвечай на вопрос!

The sentence is absolutely correct, ответь is already an imperative.

In Russian, there are perfective (совершенного вида) and non-perfective (несовершенного вида) verbs. Ответить is perfective, отвечать is non-perfective:

So:
Perfective: Infinitive «ответить», Imperative sg. «ответь», Imp. plural (or polite) «ответьте»
Non-perfective: Inf. «отвечать», Imp. sg. «отвечай», Imp. pl. «отвечайте»


Usually they can be used to show whether the action is finished or not («Я ответил на вопрос» — «I’ve answered the question»; «Я отвечал на вопрос» — «I was answering the question», maybe I didn’t give the correct answer, but at least I was saying something).

However, in the imperative mood the distinction is purely stylistic (perfective forms «ответь», «ответьте» are more polite than the non-perfective forms «отвечай», «отвечайте»), so the change «ответь» → «отвечай» you’re suggesting is simply changing a more polite form to a less polite one.

Well, the fact is, singular 2nd-person forms are not necessarily impolite. They are impolite when addressing a person you don’t know well, but they are OK when addressing friends or relatives. So there’s no need changing it to a ruder form.

Thank you very much for the explanation. I know the perfect and imperfect aspects in Russian, but I didn't know that it was already in the imperative (apparently, I was not familiar with the full conjugation for the perfect aspect of otvetit'). Again, many thanks!

Извините, что на русском, но, если честно, я бы и сам скорее сказал бы "отвечай".