Which precaution helps protect the water supply while mixing and loading?

The correct choice emphasizes the importance of ensuring that the water being used for mixing and loading pesticides is free from contamination. Running water through the hose for 30 seconds before use helps flush out any residual chemicals or substances that may have accumulated in the hose. This routine helps maintain clean water flow, which is crucial in protecting the water supply from potential pollutants from previous pesticide mixing operations.

By taking this precaution, you minimize the risk of introducing harmful substances into the water supply, which is a critical consideration in pest control practices. It's essential to ensure that any equipment or hoses used do not contaminate the water source, thus preserving the integrity of local water systems.

The other options do not effectively address the protection of the water supply in the context of mixing and loading pesticide formulations. For example, leaving equipment unattended can lead to spills or mishandling that could contaminate water sources. The absence of an air gap between the water supply and the container could prevent backflow, which indeed is a necessary practice but does not directly address flushing out contaminants. Filling containers to the top could lead to overflow, which might also result in spills and potential contamination. Thus, the choice of running water through the hose instead stands out as the most relevant and protective measure.