Which of the following best describes a plumbing plan's layout?

A plumbing plan's layout is best described by how it visually represents the arrangement of plumbing fixtures, pipes, and systems within a building. This layout is crucial for understanding the overall plumbing system, including how different components are connected and where they are located relative to each other.

By illustrating the plumbing fixtures—such as sinks, toilets, and showers—as well as the pipelines that connect them, the plan ensures that all aspects of the plumbing work together effectively. This comprehensive view helps in the planning, installation, and maintenance of plumbing systems, ensuring compliance with building codes and efficiency in water flow.

The other options do not encompass the full scope of what a plumbing plan includes. For instance, while dimensions of fixtures are important, they alone do not provide a complete layout. Similarly, details about thermal insulation pertain to a different aspect of construction altogether. Finally, focusing solely on drainage systems while ignoring water supply fails to present the entire plumbing picture, which includes both supply and waste systems. Thus, acknowledging the complete arrangement aligns with option B, highlighting its significance in plumbing design and implementation.