Which of the following is not a communication command.

Answer: Option D

In Unix, grep is not a communication command. It is a powerful utility used for searching patterns in files. It stands for "global regular expression print".
On the other hand, the other options are indeed communication commands:

write: Used to send messages to another user logged in on the same system.
mesg: Used to control whether other users can send messages to you.
mail: Used to send and receive email messages.

So, Option D: grep is not a communication command.