Backsiphonage is likely to result if:

Backsiphonage occurs when a reversal of flow in a plumbing system causes contaminated water to be drawn back into the potable water supply. This typically happens due to a drop in pressure in the potable water line, creating a vacuum that allows non-potable water to enter the system.

The scenario where a potable water line is connected to a non-potable water supply, such as for fire fighting, directly exemplifies the risk of backsiphonage. If a pressure drop occurs in the potable line (which can happen during firefighting activities when water is drawn rapidly), it creates conditions that favor the reverse flow, allowing potentially contaminated water from the non-potable supply to siphon back into the safe drinking water system.

Understanding this principle is critical in environmental health and sanitation, as preventing backsiphonage is essential to maintaining the safety and quality of drinking water. The other options involve scenarios related to pressure differences or plumbing configurations but do not directly illustrate the mechanics of backsiphonage as clearly as the connection between a potable line and a non-potable supply.