Which of the following UML diagrams is considered structural?

The use case diagram is considered a structural diagram in Unified Modeling Language (UML) as it depicts the system's functionality from an external perspective and outlines the interactions between users (actors) and the system. It provides a high-level overview of what the system does, emphasizing the relationships and roles in terms of system functionalities, rather than the flow of actions or state changes.

This diagram categorizes the elements involved—such as actors, use cases, and their interactions—allowing developers and stakeholders to visualize the system's use without delving into the internal workings or processes. In contrast, the other choices focus more on behavioral aspects: activity diagrams illustrate workflows, sequence diagrams show interactions over time, and state diagrams represent the states and transitions of a system through various events. Each of these emphasizes dynamic behavior rather than the structural relationships and functions that define the overall system architecture.