Which of the following best describes a state diagram?

A state diagram is fundamentally a behavioral modeling tool that illustrates the life cycle of a data entity by representing its various states and the transitions between those states in response to events. This visual depiction allows analysts to understand how a particular data entity evolves over time, including the conditions that trigger state changes and the actions that occur during those transitions.

By focusing on the life cycle of a data entity, state diagrams are instrumental in software design, especially in object-oriented programming, as they clarify how different states affect the system's behavior and the overall functionality of that data entity within the system.

The other options, while they relate to modeling in some capacity, do not accurately capture the essence of a state diagram. For example, a notation for DFD pertains to data flow diagrams which emphasize the transfer of information rather than states. A model to demonstrate system usability doesn't specifically address state changes, and a representation of data flow focuses more on how data moves through a system instead of its life cycle. Thus, the choice that best defines a state diagram is indeed that it shows the life cycle of a data entity.