Air distribution systems must be equipped with smoke detectors that are _____ and labeled.

The requirement for smoke detectors in air distribution systems to be "listed" is critical because it ensures that the devices meet specific safety and performance standards established by recognized testing laboratories. A listed smoke detector is one that has undergone rigorous testing and evaluation, confirming that it functions correctly in the intended application. This labeling provides assurance to installers, building owners, and occupants that the smoke detectors will perform effectively in the event of a fire, conforming to recognized safety regulations and codes. Ensuring that smoke detectors are listed helps maintain compliance with local codes and contributes to the overall safety of the building's occupants.

The other choices—approved, certified, and designed—while relevant in various contexts, do not carry the same implication of formal evaluation and assurance that "listed" does. "Approved" often signifies that a product has been accepted by a governing body, but it does not inherently guarantee that the product has passed the rigorous tests that "listed" implies. "Certified" typically refers to a product meeting certain standards but may not include specific requirements related to smoke detectors in the context of air distribution systems. "Designed" focuses more on the planning aspect rather than the validation of a product's safety or effectiveness. Hence, "listed" is the most appropriate term in