What does a network protocol define?

A network protocol defines the rules for data exchange between devices. This includes how data is formatted, transmitted, and processed, ensuring that devices can communicate effectively and understand each other. Protocols govern aspects such as error checking, message framing, address formatting, and the manner in which connections are established and terminated. This standardization is crucial for enabling different types of hardware and software to interoperate within a network.

The other options do not encompass the scope of what a protocol is. For instance, while the physical connections between devices and the types of cables used are vital to network design, they relate more to the physical layer of the OSI model rather than the communication rules themselves. Additionally, while setup costs are an important consideration for network planning, they do not pertain to the functionality or operation of a protocol. Thus, option C accurately captures the essence of what a network protocol is meant to define.