Which description best describes the execution specification in a sequence diagram?

• A. A thin, vertical rectangle that covers a lifeline during active execution
• B. A horizontal line showing message flow
• C. A box around the entire lifeline
• D. A note attached to the lifeline

Answer: (A)

In a sequence diagram, the execution specification (often called the activation) shows the period an object is actively performing work. It appears as a thin vertical rectangle on the object's lifeline, starting when the operation begins and ending when it finishes. This visual cue clearly marks the duration of active execution.

That’s why the option describing a thin vertical rectangle covering the lifeline during active execution is correct. Other elements don’t represent active processing: a horizontal arrow shows a message flow between lifelines, a box around the entire lifeline isn’t the standard notation for activity, and a note is simply an annotation attached to a lifeline.