The correct answer is: C. wall
The wall command is used to send a message to all users who are currently logged in to the system. The message is displayed on the user’s screen, regardless of which program they are currently using.
The write command is used to send a message to a single user. The message is displayed in a separate window, and the user can choose to read it or not.
The mail command is used to send an email message to a user. The message is sent to the user’s email address, and they will receive it in their email client.
The mesg command is used to control whether or not a user is allowed to receive messages from other users. The mesg y command allows messages, while the mesg n command disallows them.
E. None of the above is not a valid option.