What is a use case diagram?

A use case diagram is a crucial tool in systems analysis and design, primarily used to illustrate the interactions between users (or "actors") and the system being developed. It provides a visual representation that clearly identifies the different ways users will interact with the system, outlining specific functions or use cases. This visual clarity helps stakeholders understand the user's needs, system functionalities, and how the system will accomplish various tasks.

By depicting actors and their relationships to various use cases, the diagram highlights the system's requirements and scope. This communication tool is particularly useful during the early stages of software development to facilitate understanding among developers, project managers, and clients. Thus, option B correctly identifies a use case diagram as a visual representation of interactions between users and a system, emphasizing its role in requirement gathering and system design.