In computer networks, the primary function of a router is to:

A. connect two or more computers on the same local area network.
B. connect a computer to the Ethernet
C. connect local area networks
D. detect collisions of data packets from computers on the network and retransmit them as necessary

The primary function of a router is to C, connect local area networks together.

A router enables communication between networks by acting to direct packets of data. Any information sent over the internet is in the form of data packets. In order to go from its origin node to its destination node, there needs to be intermediate steps that possess knowledge of the destination node in order to effectively direct the packets in the correct direction. This is the function of a router, and often data packets are sent over multiple routers along their journey from start to finish. In order to make these routing decisions, these devices have dynamically updating routing tables in which they reference.