Which situation presents the highest risk of backflow?

The situation involving a hose submerged in a pool presents the highest risk of backflow due to the potential for contamination. When a hose is submerged, it can create a pathway for water from the pool, which may contain chemicals or pollutants, to be drawn back into the potable water system. This can occur if negative pressure is created in the plumbing system, allowing contaminated water to flow back through the hose and into the clean water supply.

Unlike the other scenarios, where the opportunity for contamination is either minimal or nonexistent, the submerged hose directly interacts with a body of water that is not safe for drinking. The risk here is compounded if the submerged hose is connected to a water supply without backflow prevention, making it crucial for users to be aware of the potential hazards involved in such situations. Maintaining proper backflow prevention devices and ensuring that hoses are not submerged in contaminated sources is essential for safeguarding the drinking water supply.