Which of the following allows devices on one network to communicate with devices on another network ?

Different networks often use different addressing schemes, protocols, or data formats, so a device is needed to translate between them whenever data must travel from one network to another. The device designed for exactly this role sits at the boundary between two networks and converts data from the format or protocol used on one network into the format required by the other, acting as an entry and exit point that allows devices on separate networks to communicate with each other.

A device used to connect and switch between two communication lines manually is a much simpler switching tool and does not perform the protocol translation needed to join two different networks. A device that converts digital data into signals suitable for transmission over a telephone line, and back again, connects a single computer or network to a communication line, but it does not translate between two different networks' addressing or protocol standards. A device that combines multiple separate signals into one shared communication channel, so they can be transmitted together and later separated at the receiving end, is concerned with sharing transmission capacity, not with enabling communication between two distinct networks.

The keyword to associate with cross-network communication is the device sitting at the "gate" between two different networks, translating data so that devices on each side can understand one another despite using different network standards.