Which of the following does NOT use a proxy software to protect users?

Stateful inspection tracks the state of active connections and validates packets against known session context, but it is not a proxy-based mechanism and does not intermediate connections on behalf of users.

Packet filtering evaluates packets based on header fields such as source or destination IP address and port number, with no proxy involvement and no session or content awareness.

An application layer gateway fully proxies connections at the application layer by inspecting and re-originating traffic on behalf of the user, making it a true proxy-based protection mechanism.

A circuit-level proxy server operates at the session layer (Layer 5) and verifies that TCP handshakes represent legitimate sessions, but it relays traffic without inspecting application-layer payload or acting as a content-aware intermediary on behalf of the user. Because it does not fully proxy application traffic - it only monitors the session establishment - it is not classified as proxy software that actively protects users from application-layer threats.