Which type of gateway functions in all layers of the OSI/RM?

A.
A circuit-level gateway

B.
An application-level gateway

C.
A proxy gateway

D.
A universal gateway

Explanation:
An application-level gateway operates at all levels of the OSI model. It is too high in the level to provide NAT, NAT has to be provided at a lower layer through a different service.
Note: An application-level gateway is often called an application-proxy gateway.
Sometimes it is referred to as a proxy gateway.
Incorrect Answers:
A: A circuit-level gateway operates at the session layer, and NAT is a primary advantage.
C: A proxy gateway operates at the network layer.
D: A universal gateway is not a term used to identify gateways.