Which of the following is an example of penetration testing?

Implementing a Host Intrusion Detection System (HIDS) is a defensive monitoring control that detects suspicious activity on a host, not an active simulation of an attack.

Implementing a Network Intrusion Detection System (NIDS) is a passive detective control that identifies intrusions in progress, not a proactive penetration test.

Configuring a firewall to block unauthorized traffic is a preventive security control and not an activity that simulates an attacker attempting to compromise a network.

Penetration testing, also known as ethical hacking, is defined as the authorized simulation of real attacker techniques against a target network or system to discover security weaknesses before malicious actors can exploit them. By replicating actual attack methods, testers validate whether existing controls are effective and reveal gaps in the security posture. This active, adversarial approach is what distinguishes penetration testing from passive monitoring or preventive configuration.