In a scenario-based development approach, use case diagram could best be used to describe the scenarios. A use case diagram shows how actors interact with use cases in different scenarios or situations. A use case diagram can capture the functional requirements and goals of a system or part from different stakeholder perspectives. A use case diagram can also show how use cases are related to each other by generalization, inclusion or extension relationships. A state diagram shows how an object changes its state in response to events or stimuli. A statediagram is not suitable for describing scenarios that involve multiple actors or use cases. An activity diagram shows how actions are performed in a sequence or in parallel by objects or flows. An activity diagram is not suitable for describing scenarios that involve different situations or alternatives. A requirements diagram shows how requirements are specified, derived, satisfied or verified by model elements. A requirements diagram is not suitable for describing scenarios that involve interactions or behaviors. References: OMG-Certified Systems Modeling Professional - Model Builder – Advanced (OCUP2-ADV) Examination Guide Version 1.0, Section 4.2