Gateway is Known as Protocol Converter.
ANS is D) Gateway
Why Gateway? Why dont we use Router?
Ans: There is a small difference in between router and gateway.
Router is used for connecting two network of same type, for example if you want to connect two distinct TCP/IP network, in that case Router is is useful. Because both the network is of same type.
But if there is a TCP/IP network, and now you want to connect it to an ATM network, what will you do? Here Gateway comes into picture. Gateway is the solution when you want to connect two different type of network.
How Gateway works?
As you know, router have only 3 layer, Physical Layer, Data Link Layer and Network Layer. Actually "Different Type of Network" means, different type of network packet format, and all. The differences is actually is the differences of network layer.
Now, Gateway has Physical Layer, Data Link Layer, Transportation Layer and Application Layer. The Application Layer Data is extracted using these layer once,then the RAW Data is put into respected packet format of that respected protocol, where actually you want to send that data. The goes though Transport Layer,Network Layer, Data LInk Layer and it come out in required format.