Final answer:
The most effective diagram for describing the internal steps of a use case is a workflow diagram, which provides detailed flow of activities and is essential in the engineering design process.
Step-by-step explanation:
The diagram that is most effective in describing the internal steps of a use case is a workflow diagram. While a use case diagram is good for outlining the interactions and scope of a system, a workflow diagram (also known as an activity diagram) details the internal steps and the flow of activities within a particular use case. It graphically represents the sequence of steps, decisions that need to be made, and the flow of control from one activity to the next. This is essential in the engineering design process to ensure that all aspects of the product's development follow a clear and organized path. None of the other listed diagrams, including the package diagram, use case diagram, or class diagram, provide the same level of detailed process flow that is critical to understanding and executing the use case steps.