The unobstructed vertical distance through the free atmosphere between an outlet and the rim best defines what?

The correct answer is an air gap. An air gap is defined as the unobstructed vertical distance between the lowest opening of a fixture and the level of the liquid in the receptacle into which it discharges. This distance acts as a physical separation that prevents backflow, ensuring that contaminated water cannot siphon back into the potable water supply. The purpose of an air gap is to eliminate the risk of cross-connection between potable water and non-potable sources, thus protecting public health.

In contrast, a vent pipe is utilized to allow air to enter the drainage system to maintain equalized pressure and facilitate proper drainage, but it does not serve the purpose of preventing backflow by creating a physical separation. A drainage slope refers to the angle at which pipes are installed to ensure adequate flow of wastewater, rather than defining any specific distance between fixtures. A backflow preventer is a device designed to stop backflow but does not pertain to a distance or physical separation, as an air gap does.