A circuit level gateway works at which of the following layers of the OSI Model?

Choice A mislabels Layer 5 as the Application layer - the Application layer is Layer 7; Layer 5 is the Session layer - and circuit-level gateways operate below that at the TCP/Transport layer.

Circuit-level gateways operate at Layer 4 (Transport layer) of the OSI model, where they monitor TCP handshake messages such as SYN, SYN-ACK, and ACK to verify that a session is legitimate before permitting traffic. They do not inspect the actual application-layer content, which distinguishes them from application-layer (proxy) firewalls. This TCP connection-state validation is the defining operational characteristic of a circuit-level gateway.

Layer 3 is the Network layer, which handles IP addressing and routing; circuit-level gateways function above this layer and focus on TCP connection state rather than packet routing.

Layer 2 is the Data Link layer, responsible for MAC addressing and frame switching; circuit-level gateways operate well above this layer and have no involvement in frame-level processing.