What operates at the session layer of the OSI model?

The session layer of the OSI model is responsible for establishing, managing, and terminating connections between applications. It ensures that sessions are properly maintained, allowing for the orderly and synchronized communication between systems.

A circuit-level gateway operates at the session layer because it functions by establishing a session between two endpoints before transmitting data. This type of gateway can monitor and control the session, ensuring that both sides maintain the connection, which aligns closely with the functions defined at the session layer.

In contrast, other options represent technologies that serve at different layers of the OSI model or do not focus primarily on session management. Network Address Translation, for example, is primarily concerned with translating private IP addresses into public addresses and managing the routing at the network layer. Application Level Gateways focus on inspecting traffic at the application layer to enforce security protocols, and Virtual Private Networks generally create secure connections through encryption and tunneling, often functioning at the transport layer and above rather than strictly at the session layer.

Thus, the circuit-level gateway is the only option that is specifically aligned with the responsibilities of the session layer, making it the correct choice in this context.