What is one component that must be labeled according to radon system requirements?

One component that must be labeled according to radon system requirements is the duct piping near an Active Soil Depressurization (ASD) fan. This labeling is important for several reasons. It helps to clearly identify the components of the radon mitigation system to homeowners and professionals involved in maintaining or inspecting the system. Clear labeling ensures that individuals can easily locate the system's parts that are crucial for proper functioning, which can prevent accidental damage during renovations or maintenance activities.

Moreover, labeling the duct piping near the ASD fan is essential for safety. It allows for efficient monitoring and operation of the radon mitigation system, ensuring that it is functioning correctly to reduce radon levels effectively. When individuals see the labeled components, they are reminded of the system's purpose and importance in protecting indoor air quality by reducing radon exposure.

In contrast, the other options do not pertain directly to the labeling requirements set forth by radon system standards. Electrical outlets, building foundations, and paint are not components that require specific labeling related to radon mitigation and do not play a direct role in the system's performance or maintenance.