Which type of device communicates with external servers on behalf of internal clients?

A.
A client-level gateway

B.
An application-level gateway

C.
A proxy server

D.
A packet filter

Explanation:

The key word here is behalf. The proxy server is a server used to protect networks, and is usually placed between a corporate network and the Internet. Requests sent to the proxy server are re-issued by the proxy server on behalf of the client, and externally the requests have the IP address of the proxy server.
Incorrect Answers:
A: A gateway is a device that does protocol conversion at layer 4 of the ISO model. It does protocol conversion, but does not act in behalf of any clients passing traffic through the gateway.
B: A gateway is a device that does protocol conversion at layer 4 of the ISO model. It does protocol conversion, but does not act in behalf of any clients passing traffic through the gateway.
D: A packet filter operates at Layer 3 of the ISO model, and inspects packet traffic and either allows or drops the packet after inspection. Even in the case of stateful inspection, packets are inspected and either allowed or denied. The packet filter, usually in a firewall, does not act on behalf of any client, it is just a censor of the datastream.