The correct answer is D. SMTP.
SMTP stands for Simple Mail Transfer Protocol. It is a standard protocol for sending email messages between servers. SMTP is a text-based protocol, which means that it uses plain text to send and receive messages.
FTP stands for File Transfer Protocol. It is a standard protocol for transferring files between computers. FTP is a binary protocol, which means that it can transfer any type of file, including binary files.
SSH stands for Secure Shell. It is a cryptographic network protocol for operating network services securely over an unsecured network. SSH provides a secure channel over an unsecured network in a client-server architecture, connecting an SSH client application with an SSH server.
POP3 stands for Post Office Protocol version 3. It is a standard protocol for retrieving email messages from a mail server. POP3 is a text-based protocol, which means that it uses plain text to send and receive messages.
I hope this helps!