A firewall implements ACLs (Access Control Lists) to filter traffic and enforce policies based on rules such as IP address, port, or protocol. * Why Not A (Managed switch): A managed switch supports VLANs and QoS but does not enforce ACLs at the network perimeter. * Why Not B (Gateway): Gateways connect networks and translate protocols but don't typically implement ACLs. * Why Not C (Repeater): Repeaters extend signal range but don't enforce policies. CompTIA A+ Exam Reference: Core 1 (220-1101), Section 2.2, firewall concepts.