noun
reply letter; written response
A formal or official letter written in reply to an inquiry, request, or previous correspondence.
お問い合わせに対する返答書を本日発送いたしました。
We have sent the reply letter regarding your inquiry today.
返答書には、会議の日程について詳しく書かれていた。
The written response detailed the meeting schedule.
回答書 is a written answer to a question or survey, whereas 返答書 is a reply to a letter or request.
Compound of 返答 (reply) + 書 (document, letter).