Answer -> 一般的な『答え』

They usually don't answer when they're busy.

=彼らは忙しい時は電話に出ないです。

That’s a difficult question to answer.

=それは答えづらい質問ですね。

Reply -> 手紙などに対する返事

I don’t know how to reply to that.

=どう返事すればいいか分からない。

I am still awaiting his reply.

=私はまだ彼の返事を待っているのだ。

Response -> 問いかけに対する反応

Expecting a response to this letter.

=ご返信お待ちします。

Thank you for your quick response.

=早急なお返事ありがとうございます。

「テスト」

間違いやすい例

X All of the replies on the test were wrong.

X All of the reponses on the test were wrong.

正しい例

O All of the answers on the test were wrong.