Which UML diagram can show a user’s journey during the system’s usage?

The use case diagram is particularly well-suited to illustrate a user's journey during a system's usage because it captures the interactions between users (or actors) and the system in terms of specific goals or use cases. In a use case diagram, each use case represents a functional requirement of the system from the user's perspective, showing what the user can do and how they can engage with the system to achieve particular objectives.

This diagram helps stakeholders understand the user experience by visualizing the flow of interactions, including both the primary actions users take and the intended outcomes of those actions. Overall, it effectively outlines the scenarios in which users interact with the system, highlighting the user's journey throughout their engagement with it.

In contrast, class diagrams focus on the structure of a system in terms of classes and their relationships, sequence diagrams illustrate how objects interact in a specific sequence, and activity diagrams display the flow of activities within a process without necessarily emphasizing user involvement as the primary focus. Each of those diagrams serves different purposes and does not provide the same direct insight into the user's journey as the use case diagram does.