A firewall monitors and filters incoming and outgoing network traffic based on security rules, protecting the network from unauthorized access and threats. * Why Not A (Switch): A switch connects devices within a network but does not monitor or filter traffic. * Why Not B (Access point): An access point provides Wi-Fi connectivity but does not filter traffic. * Why Not D (Hub): A hub simply broadcasts data to all connected devices without filtering traffic. CompTIA A+ Exam Reference: Core 1 (220-1101), Section 2.2, network security concepts.