(something that you say, write or do to react to a question): She sent an answer to my letter after a few days.
reply (to say, write, or do something as an answer to somebody): We asked why, but they gave no reply.
response (a spoken or written answer): The decision in fact has received a positive response.
comeback (a quick clever, funny, or rude reply): Gabriel walked out before I could think of a witty comeback.
acknowledgement (a message sent to say that something has been received): I’m writing him but never get any acknowledgement.