Final answer:
The correct answer is 'd) Indirect waste,' which refers to the unobstructed vertical distance called an air gap in a plumbing system, ensuring no backflow or cross-contamination occurs in the potable water supply.
Step-by-step explanation:
The unobstructed vertical distance through which the free atmosphere between the lowest opening from any pipe or faucet conveying water to a tank, plumbing fixture, receptor, or other assemblies and the flood level rim of the receptacle is known as the air gap. This air gap prevents contaminated water from being drawn back into the potable water supply in the event of a negative pressure event, which could cause backflow contamination. Therefore, the correct answer to the question is 'd) Indirect waste.' Indirect waste systems utilize an air gap to ensure that any waste water or substances cannot contaminate the potable water supply due to backflow.
Moreover, the presence of air-filled tubes near water faucets is also to prevent backflow and cross-contamination, ensuring that clean water is maintained within the plumbing system. Such systems are critical in maintaining water quality and are a standard feature in modern plumbing to protect health and well-being.