What is a lifeline in a sequence diagram?

A lifeline in a sequence diagram is correctly defined as a rectangle placed at the top of a dashed line. This structure represents an individual participant in the interaction, such as an object or an actor. The dashed line that extends downward from the rectangle indicates the existence of that participant over time during the interaction. The lifeline is a crucial aspect of sequence diagrams, as it helps to visualize the timeline of interactions between different entities and represents when an object is active or inactive during the sequence of events.

Understanding the representation of lifelines is important for modeling the flow of control and communication between various components in a system, as it helps to clarify how messages are exchanged over time. The other options do not capture the true essence of a lifeline, as they describe different elements or concepts within a sequence diagram rather than focusing on the lifeline's specific characteristics and functions.