Which of the following best describes an Air Gap in water distribution?

An air gap in water distribution is best described as the distance between any water supply opening and the flood rim of a fixture. This vertical separation is critical in preventing contamination of the potable water supply by ensuring that there is no direct connection between the water supply and any potentially contaminated water. For instance, if water were to backflow into the supply line from a sink or a toilet, the air gap would ensure that the contaminated water cannot rise high enough to mix with the clean water. This concept is foundational in plumbing to maintain safe drinking water and is crucial for the prevention of cross-contamination in plumbing systems.

The other options refer to different concepts unrelated to the definition of an air gap. A vacuum created by a mechanical device pertains to the mechanics of pressure differences, while the capacity of a pump to move water relates to the mechanical performance of pumping systems. An opening for venting sewer gases is distinctly associated with plumbing vents, not with water supply mechanisms.