Which of the following describes the function of a router?

The function of a router is primarily to determine the best path across a network for data packets. Routers are essential components in computer networking, designed to route traffic efficiently between various devices and networks. They analyze the destination IP address of packets and utilize routing tables and protocols to figure out the optimal way to send these packets onward to their destinations. This process is crucial in maintaining effective communication and data transfer over the internet or within a private network.

The other options describe different types of network devices or functions. Transmitting signals over radio frequencies pertains more to wireless access points or radios, while temporarily storing data for quick access is a characteristic feature of cache memory or intermediate devices like switches and gateways that may have local memory. Encrypting communication is a function associated with security protocols and devices like VPNs or firewalls, rather than routers themselves. Thus, the accurate portrayal of the router's role is in its routing capabilities, making option C the correct choice.