Which of the following is primarily used to connect different networks?

The correct answer is D. Router.

According to the CompTIA A+ Core 1 (220-1201) objectives for Networking, routers are devices that operate at Layer 3 (Network layer) and are primarily used to connect multiple networks, including LANs, WANs, and the Internet. Routers use IP addressing to determine the best path for forwarding packets between networks.

Why the other options are incorrect:

A. Controller: Generally refers to a device that manages a network or hardware component but does not inherently connect different networks.

B. Bridge: Connects two segments of the same network, not different networks, and operates at Layer 2 (Data Link layer).

C. Hub: Connects devices within the same network segment, simply repeating signals, and does not perform routing between networks.