The Post Office Protocol version 3 (POP3) is a standard application-layer protocol used by email clients to retrieve email from a mail server. The Internet Assigned Numbers Authority (IANA) has officially assigned TCP port 110 as the well-known port for the POP3 service. An investigator using a network protocol analyzer like Ethereal (now Wireshark) would apply a display filter for tcp.port == 110 to isolate and analyze unencrypted POP3 traffic, which includes commands, responses, and email content.

A. 143 is the standard port for the Internet Message Access Protocol (IMAP), another email retrieval protocol, not POP3.

B. 25 is the standard port for the Simple Mail Transfer Protocol (SMTP), which is used for sending (submitting) email, not retrieving it.

D. 125 is not the standard port for POP3. It is unassigned by IANA for any common email protocol and is sometimes used for other services.

1. Internet Assigned Numbers Authority (IANA). (2024). Service Name and Transport Protocol Port Number Registry. IANA. Retrieved from https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml. The registry explicitly lists "pop3" assigned to TCP port 110.

2. Myers, J., & Rose, M. (1996). RFC 1939: Post Office Protocol - Version 3. Internet Engineering Task Force (IETF). In Section 3, "Basic Operation," it states, "The POP3 server listens on TCP port 110." DOI: https://doi.org/10.17487/RFC1939.

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. Chapter 2, Section 2.4.1, "SMTP," and Section 2.4.2, "Mail Access Protocols," detail the port numbers for email protocols, specifying port 25 for SMTP, 110 for POP3, and 143 for IMAP. This is a standard textbook used in many university computer science programs.