What shape denotes a pin on an action boundary?

• A. Small square
• B. Small circle
• C. Diamond
• D. Triangle

Answer: (A)

Pins on an action boundary show where data enters or leaves an action in an activity diagram. The shape used for the pin signals its role: a small square marks an input pin, indicating data flowing into the action. This makes it easy to trace how inputs are provided to the action. A small circle would indicate an output pin, showing data produced by the action. Other shapes like a diamond or triangle are used for other diagram elements, not pins. So the small square is the symbol used to denote a pin on an action boundary.