Which model element is specifically represented by a state machine diagram?

A state machine diagram specifically represents a state machine, which is a model element that describes the states of an object and the transitions between those states in response to events. This type of diagram is particularly useful for depicting dynamic behaviors of systems and showing how a particular entity or object reacts over time to different stimuli or events.

State machine diagrams help in visualizing the different states that an object can occupy, along with the conditions that trigger transitions from one state to another. They are helpful in understanding the lifecycle of an object, especially in complex systems where the behavior can vary significantly depending on the current state.

In contrast, while blocks, classes, and activities can be represented in various modeling diagrams, they do not specifically convey the same focus on state transitions and behaviors over time that state machine diagrams do. This distinction underlines why a state machine is the correct model element represented in this context.