Which of the following device is used to connect two system, especially if the system use different protocols?

Explanation:

A gateway device is used to connect two systems, especially when the protocols are incompatible. A network switch, as opposed to a less advanced network hub, sends data only to one or more devices that require it, rather than broadcasting the same data out of all of its ports. It operates at OSI layer 2.

Final Answer:

Hence, Option (A) is correct i.e. Gateway.