Synonyms: answer, reply, response, acknowledgement
answer: something that you say or write when someone has asked you a question
Example: The answer to your question is yes.
reply: an answer, especially to a letter or telephone call
Example: We wrote last week, but haven’t had a reply yet. We had six replies to our advertisement.
response: something that you do or say as a reaction to something
Example: There was no response to our call for help. The changes produced an angry response from customers.
acknowledgement: a letter or note sent to say that something has been received
Example: We didn’t even receive an acknowledgement from the council. a letter of acknowledgement
a sentence which needs an answer