Which of the following would a network administrator MOST likely use to actively discover unsecure services running on a company's network?

An IDS (Intrusion Detection System) passively monitors traffic for attack signatures and anomalies; it does not actively probe the network for vulnerable services.

Nessus is an active vulnerability scanner that connects to discovered services, tests them against a database of known vulnerabilities, and reports misconfigurations and insecure services. This directly addresses the requirement to actively discover unsecure services across the network.

NMAP is primarily a port scanner and OS fingerprinting tool that identifies open ports and running services but does not assess whether those services are insecure or vulnerable.

A firewall enforces access control policies to block unauthorized traffic; it does not scan or report on the security posture of internal services.