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