Backflow occurs when:

Backflow refers to the unintended reverse flow of water, which can lead to contaminants entering the clean water supply, posing significant public health risks. This phenomenon generally occurs when there is a drop in pressure within the water distribution system, which can create conditions conducive to backflow. When the pressure decreases, it may allow water to flow backward through connections in the plumbing system that should only allow water to flow in one direction.

Choice C accurately captures this concept, as it emphasizes that contaminants can indeed flow back into the water supply during backflow events. This can happen through a variety of mechanisms, including cross-connections where dirty water and clean water systems are improperly connected.

While acknowledging other relevant aspects, such as the importance of maintaining proper pressure in water supply systems and the need for backflow prevention measures, the focal point remains on how backflow introduces contaminants, which is essential for understanding health risks related to water safety and sanitation practices.