Back-siphoning refers to:

Back-siphoning is a term that specifically describes the undesirable process where liquid, often containing pesticides, is drawn back into a water supply. This can happen if there is a sudden drop in water pressure, leading to a reverse flow from a pesticide application device into the drinkable water system. This contamination poses significant health risks, which is why understanding and preventing back-siphoning is critical in pest control practices.

Preventive measures are often established, such as the use of backflow prevention devices, to ensure that pesticide solutions don't enter potable water systems. Therefore, the correct answer highlights the specific risk associated with this phenomenon in relation to water safety and pesticide management.

The other options do not accurately define back-siphoning; for instance, preventing pesticide exposure focuses on safety measures, recycling empty pesticide containers is an environmental concern, and using siphons to distribute pesticides refers to a method of application rather than the risk of back-siphoning itself.