How is a message represented in a sequence diagram?

In a sequence diagram, a message is represented by a line with an arrowhead. This graphical element indicates the direction of communication between lifelines, showing which participant sends the message and which participant receives it. The arrowhead is crucial as it visually conveys the flow of control or data in the interaction between the objects or participants represented by the lifelines.

The message itself can also include text that describes the action being taken or the information being transmitted, further clarifying its purpose within that specific interaction flow. This representation helps viewers of the diagram easily understand the sequence of events and how different objects interact over time in a system.

While other representations like solid lines or dashed lines between lifelines may convey different types of relationships or interactions, they do not directly symbolize messages as clearly as the line with an arrowhead does. The clarity and explicitness of the arrowhead in conveying directionality is what makes this choice the correct representation of a message in a sequence diagram.