Explanation:

Ans: D
Both (a) and (b)

Communication software is used to provide remote access to systems and exchange files and messages in text, audio and/or video formats between different computers or users. This includes terminal emulators, file transfer programs, chat and instant messaging programs, as well as similar functionality integrated within MUDs. The term is also applied to software operating a bulletin board system, but seldom to that operating a computer network or Stored Program Control exchange.