Is you going to say to a friend??? If it is you can use "나한테 답장(or 대답)한게 넌인줄 알았어." or "네가 답장(or 대답)한 줄 알았어". You can choose 답장 and 답변. 답장 is who reply you as a text kind of letter, message, e-mail and so on. If someone reply you as a speak, 대답 will be okay.