Which of the following diagrams is used to describe the flow of an activity or a set of activities?

The activity diagram is specifically designed to illustrate the flow of control and the sequence of activities within a process. It effectively visualizes the dynamic aspects of a system by showcasing how various activities interact with each other and in what order they occur. This type of diagram is particularly useful in modeling complex workflows, enabling stakeholders to see not only the steps involved but also the decision points and interactions that may arise during the execution of those activities.

In the context of software development, activity diagrams can represent everything from business processes to the specific interactions required in a programming context. They help identify the sequence and conditions under which certain activities are performed, making them an essential tool for both analysis and design phases.

The other diagrams mentioned serve different purposes: a class diagram illustrates the structure of a system by showing the relationships between classes, a sequence diagram details the interactions between objects over time, and a use case diagram represents the functional requirements of a system from a user perspective. Each of these has a unique role, but when it comes to mapping out the flow of activities, the activity diagram is the most appropriate choice.