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

Layer 5 as labeled in the choices corresponds to the Application tier, which is the domain of application-layer proxy firewalls that perform full content inspection - not circuit-level gateways.

Circuit-level gateways function at the Transport layer where TCP operates, validating that connections conform to proper TCP handshake sequences (SYN, SYN-ACK, ACK) before permitting traffic to pass through. They relay TCP connections and monitor handshaking state without analyzing the application-layer content of the data stream. This distinguishes them from deeper application-layer proxy firewalls while providing more session awareness than simple network-layer packet filters.

Layer 3 (Internet Protocol/Network layer) is where packet filtering firewalls operate by inspecting IP headers and addresses, not where TCP session handshake monitoring occurs.

Layer 2 (Data Link) handles MAC addressing and local frame delivery and has no involvement in TCP session state monitoring performed by circuit-level gateways.