Storyboards are a visual tool used in Agile methodologies to represent and analyze user stories and epics. They provide a sequential depiction of interactions, which aids in understanding and categorizing functionalities.

Visualization of Stories: Storyboards offer a visual representation of user interactions, making it easier to categorize stories into different functional areas. This visualization helps teams understand the flow and grouping of functionalities within the system.

Categorization into Functional Areas: By depicting user interactions and system responses, storyboards assist in organizing user stories and epics into specific categories or modules. This categorization is crucial for planning development and testing activities.

Therefore, option D accurately reflects the benefit of using storyboards in analyzing user stories and epics.