Which shape indicates control logic in a combined fragment?

The shape that indicates control logic in a combined fragment is a pentagon. In the context of Unified Modeling Language (UML) diagrams, particularly interaction diagrams such as sequence diagrams, various shapes are employed to represent different constructs and elements. The combined fragment specifically depicts control structures, such as alternatives and loops, which dictate how the enclosed interactions should be executed based on certain conditions.

The pentagon shape is used to represent control logic due to its distinct geometric form, which clearly distinguishes it from other elements in the diagram. Understanding this representation is crucial for effectively interpreting the flow of interactions, as it allows the model user to identify where the behavior changes based on specific conditions or decisions.

In contrast, the other shapes mentioned do not hold significance for control logic within combined fragments. Rectangles are typically utilized to represent interactions or lifelines, circles may denote events or states but are not specific to control logic in this context, and squares likewise do not serve the purpose of indicating control logic. Recognizing the use of the pentagon shape is therefore essential for a clear understanding of complex interaction models.