Which plan diagrams the arrangement of rooms and furniture at a set scale?

A floor plan is a crucial architectural drawing that illustrates the layout of rooms, spaces, and the arrangement of furniture within a building at a specific scale. This type of plan provides a bird's-eye view of the interior layout, allowing for an understanding of how different areas relate to each other spatially. It typically includes walls, doors, windows, and furniture placements, which helps in visualizing the flow and functionality of the interior spaces.

In contrast, other types of plans serve different purposes. A foundation plan focuses on the structural elements of the building's foundation. An elevation plan provides a vertical view of the building's exterior, showing height dimensions and design details, often lacking interior arrangements. A site plan depicts the entire property layout, including the building and its relationship to the surrounding land but doesn't provide detailed insights into the internal room configuration or furniture. Thus, a floor plan uniquely serves the purpose of detailing room arrangements and furnishing layouts within a building, making it the correct answer in this context.