Final answer:
The Reference network model describes the functions that must be completed at a particular layer, but does not specify exactly how each protocol should work.
Step-by-step explanation:
The correct answer to the question is c) Reference. The Reference network model describes the functions that must be completed at a particular layer, but does not specify exactly how each protocol should work. It serves as a framework or guide for the development of network protocols. For example, the OSI (Open Systems Interconnection) model is a well-known reference model used to understand the different layers and their functions in a network.