Answer is option C: TCP.
There are three primary TCP/IP protocols for E-Mail management:
- Post Office Protocol (POP)
- Simple Mail Transfer Protocol (SMTP)
- Internet Message Access Protocol (IMAP)
They all are Application Layer Protocols
Once a client connects to the E-mail Server, there may be 0(zero) or more SMTP transactions. If the client has no mail to send, then there are no SMTP transactions. Every e-mail message sent is an SMTP transfer.
SMTP is only used to send (push) messages to the server. POP and IMAP are used to receive messages as well as manage the mailbox contents(which includes tasks such as deleting, moving messages etc.).