What defines the unobstructed vertical distance between a water supply opening and the flood level rim of a receptacle?

The correct answer is the air gap. An air gap is defined as the unobstructed vertical distance between a water supply opening and the flood level rim of a receptacle, such as a sink or bathtub. This space is critical for preventing backflow, which can occur when wastewater siphons back into the potable water supply.

By maintaining an air gap, any potential contaminants that might enter the receptacle cannot reach the water supply opening, ensuring the safety and integrity of the drinking water. This is a standard plumbing practice designed to protect public health, and its significance is underscored by plumbing codes which often require specific air gap distances based on local regulations.

The other options, while related to plumbing concepts, do not accurately describe this particular definition. Drainage height generally refers to the effective height of drainage pipes, water clearance could refer to other types of clearances in plumbing but not specifically to the distance required for backflow prevention, and pressure differential relates to the difference in pressure that might allow for flow or backflow in systems but does not describe the vertical distance involved.