Final answer:
The device that controls the flow of data and acts as a gateway between networks is called a router. It is designed to manage and forward data packets using IP addresses to find the most optimal path.
Step-by-step explanation:
A router is a Data Communication Equipment (DCE) that controls the flow of data within a network and acts as a gateway from one network to another. Among the options provided (A) Modem, (B) Router, (C) Hub, and (D) Switch, the correct answer is B) Router. Routers are designed to connect multiple networks together and manage the data traffic between them by using IP addresses to determine the best path for forwarding the packets. Unlike hubs, which simply relay signals to all devices, or switches, which facilitate data transmission within the same network, routers are much more sophisticated and play a critical role in the functioning of the Internet.