What is the least suitable means to document details of a single use case?

The least suitable means to document the details of a single use case is a context diagram. A context diagram is primarily used to show the relationships between a system and external entities, illustrating how the system interacts with these entities at a high level. It focuses on the system's boundaries and external inputs and outputs, rather than providing in-depth details about specific use cases.

In contrast, other options such as natural language are ideal for capturing the narrative of a use case, activity diagrams are effective in visualizing the flow of actions within the use case, and state diagrams can demonstrate the states that the system may enter as various events occur. Therefore, a context diagram does not adequately cover the specifics of a single use case, which requires more detailed documentation of the steps, decisions, and interactions involved.