A firewall is the device class associated with stateful inspection. Stateful inspection tracks active sessions and uses connection state when deciding whether to permit return traffic or block unexpected packets. For example, if an internal host initiates a TCP session, the firewall can allow matching return traffic because it belongs to a known connection. A Layer 2 switch forwards frames based on MAC addresses and does not inspect transport-layer session state. An access point bridges wireless clients to the wired network, and a wireless controller manages AP behavior and WLAN policy, but neither is the generic stateful-inspection answer in this question. Cisco CCNA 200-301 v1.1 Security Fundamentals requires candidates to know common security device roles: firewall, WSA, AAA server, VPN headend, and access-layer controls. The key phrase is stateful inspection of traffic. That points directly to a firewall. The correct answer is A.