A systems administrator needs to see if a mail server is listening on port 25. Which of the following is the BEST command for the administrator to run on the mail server to see if the port is…

The netstat command with flags such as -tulnp or variants thereof displays all active TCP and UDP sockets along with their listening state and the associated process. Piping the output through 'grep 25' filters the results to show only lines referencing port 25, confirming whether the mail service is actively listening. This is the standard approach for local port-state verification on a Linux host.

The telnet command tests outbound connectivity to a remote host and port but does not inspect the local socket table to confirm a port is in a listening state; it also uses incorrect flag syntax for port specification.

'ps aux' lists running processes and their arguments, not open network ports - grepping for '25' would match any process ID or argument containing that number, not reliably identify port 25.

nslookup is a DNS resolution utility used to query name servers and has no capability to inspect TCP/UDP port states on the local or remote host.