Both "Answer" and "Reply" have the same general meaning and can be chosen between. For example: "I replied back." "I answered back." Though they do have a slight difference; "reply" meaning that you respond to someone making conversation with you whilst "Answer" is when you respond to someone's question.

Definitions of answer: noun a statement (either spoken or written) that is made to reply to a question or request or criticism or accusation Example: he wrote replies to several of his critics a statement that solves a problem or explains how to solve the problem Example: the answers were in the back of the book|he computed the result to four decimal places the speech act of replying to a question verb react verbally Example: answer the question|We answered that we would accept the invitation respond to a signal Example: answer the telephone give the correct answer or solution to Example: answer the riddle Definitions of reply: noun a statement (either spoken or written) that is made to reply to a question or request or criticism or accusation Example: he wrote replies to several of his critics the speech act of continuing a conversational exchange verb react verbally Example: answer the question|We answered that we would accept the invitation

they are synonyms but slight differenc in uses answer is a spoken or written reply there is no answer for your question. i answered the question/door/telephone reply to something, also in writing or speech i'll write a reply to his question. i replied to his letter i said nothing in reply check this: www.thefreedictionary.com