What is the primary function of a sequence diagram?

The primary function of a sequence diagram is to show the timing of messages in a use-case scenario. This type of diagram is part of the Unified Modeling Language (UML) and is primarily used in software engineering to depict how objects interact in a particular scenario over time.

Sequence diagrams represent actors (users or other systems) and the objects in a system. They illustrate the sequence of messages exchanged among these components and the order in which they occur. This allows developers and stakeholders to visualize the interactions and communications that take place, making it easier to understand the dynamics of a use-case scenario in detail.

The other options pertain to different aspects of system modeling: defining class relationships is related to class diagrams; illustrating state transitions involves state machine diagrams; and representing system deployment is the focus of deployment diagrams. Each of these serves a different purpose within the broader context of system design and analysis.