Final answer:
A Switch is the device that primarily acts as a tool to improve network efficiency by directing data only to the intended recipient, reducing unnecessary traffic on the network.
Step-by-step explanation:
The device that acts primarily as a tool to improve network efficiency is a Switch. Unlike a hub, which broadcasts the same data to all ports, a switch smartly directs data to the intended recipient by learning the addresses of the devices on the network. This significantly reduces unnecessary traffic and increases network efficiency.
In contrast, a router connects multiple networks and directs data between them, while a PBX (Private Branch Exchange) is a telephone system within an enterprise that switches calls between enterprise users on local lines while allowing all users to share a certain number of external phone lines.