"Why haven't they replied yet?"
回答 means answer, and 回复 means reply.
Searched from the web: Both "Answer" (回答) and "Reply" (回复) have the same general meaning and can be chosen between. 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.
The better one here is 回复.