Which application layer protocol is used to retrieve

Solution

POP3 (Post Office Protocol version 3) is an application layer protocol used by email clients to retrieve messages from a mail server. POP3 downloads the emails from the server to the client and typically deletes them from the server, though this behavior can be modified in some configurations. POP3 is used primarily for receiving emails. Option A (FTP) is incorrect because FTP is used for transferring files between computers, not for retrieving emails. Option B (SMTP) is incorrect because SMTP is used for sending emails, not retrieving them. Option D (HTTP) is incorrect because HTTP is used for accessing websites and web services, not for retrieving emails. Option E (SNMP) is incorrect because SNMP is a protocol for managing and monitoring network devices, not for handling emails.