Final answer:
The correct TCP/IP protocol for email transmissions is SMTP, which stands for Simple Mail Transfer Protocol. It is essential for sending emails over the Internet and works in conjunction with MTAs, MSAs, and MDAs to route and deliver emails.
Step-by-step explanation:
The TCP/IP application-layer protocol used for email transmissions is C. SMTP (Simple Mail Transfer Protocol). SMTP is the standard protocol for sending emails across the Internet. It works closely with something called the Mail Transfer Agent (MTA) to send your communication to the right computer and email inbox.
SMTP moves your email on and across networks. It works closely with Mail Submission Agents (MSA) and Mail Delivery Agents (MDA) to deliver the emails to the recipient's local mail server. SMTP forwards the email on from the sender's mail server to the recipient's mail server. Pop3 or IMAP are used to download the email to the recipient's machine for reading.