Which network device monitors incoming and outgoing traffic and decides whether to allow or block specific traffic based on a defined set of rules?

A switch operates at Layer 2 and forwards frames based on MAC address tables; it does not inspect or enforce security policies on traffic content.

A load balancer distributes incoming requests across multiple backend servers to optimize utilization and availability; its primary purpose is not to enforce traffic security policies.

A reverse proxy forwards client requests to backend servers and may provide SSL termination or caching, but its primary function is request routing and proxying, not allow/deny enforcement based on a security rule set.

A firewall evaluates network packets against a defined policy rule set - which may use ACLs, stateful inspection tables, or application-layer signatures - to permit or block traffic. This inspection-and-enforcement function is the fundamental and defining characteristic of a firewall, distinguishing it from all other network devices.