A firewall is a fundamental information security control designed to monitor, filter, and control incoming and outgoing network traffic based on predefined security rules. This makes option A the correct answer.

Firewalls act as a barrier between trusted internal networks and untrusted external networks, such as the internet. They can be implemented as hardware devices, software applications, or cloud-based services. Ethical hackers must understand firewall behavior because it directly affects reconnaissance, exploitation techniques, and attack surface visibility.

Option B is incorrect because antivirus software focuses on malware detection, not traffic filtering. Option C is incorrect because a firewall is a defensive security mechanism, not an attack method.

From an ethical hacking perspective, firewalls are evaluated during security assessments to identify misconfigurations, overly permissive rules, or exposed services. Poorly configured firewalls may allow unauthorized access, while overly restrictive ones may disrupt legitimate business operations.

Firewalls play a critical role in enforcing network segmentation, access control, and defense-in-depth strategies. When combined with intrusion detection systems, endpoint security, and proper monitoring, they significantly reduce the risk of unauthorized access.

Understanding firewall concepts enables ethical hackers and defenders to design stronger network architectures and respond effectively to modern cyber threats.