The correct answer is D. SMTP.
SMTP stands for Simple Mail Transfer Protocol. It is a standard network protocol used for sending email messages between servers. SMTP is an application-layer protocol that is typically used over TCP/IP.
FTP stands for File Transfer Protocol. It is a standard network protocol used for transferring files between computers. FTP is a transport-layer protocol that is typically used over TCP/IP.
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 an Internet standard protocol for accessing e-mail from a remote server. POP3 is a client-server protocol that is typically used over TCP/IP.
In summary, SMTP is the protocol used to send email messages over the internet. FTP is used to transfer files between computers. SSH is used to create a secure connection between two computers. POP3 is used to retrieve email messages from a remote server.