Hi, 대답 [對答] is a noun that means one's reply or response. It's verbal form 대답하다 [對答하다] means to reply or respond.
답 is a noun that also means one's reply or response. However, a verb phrase such as 답하다 does not exist in the Korean language.
답 can also be found in phrases such as 답안 [答案], a noun which means an answer, as well as 회답 [回答] which means a reply.
Good luck in your studies!
-Raymond