Which of the following describes an engineering control for handling pesticides?

The choice of a closed mixing system as an engineering control for handling pesticides is appropriate because it directly addresses the need to reduce exposure to pesticides during the mixing process. Engineering controls are designed to isolate or remove a hazard from the workplace, and closed mixing systems do this by preventing the release of pesticide vapors and spills into the environment. These systems often involve mechanisms that allow for the accurate mixing of pesticides without the need for manual handling, thereby minimizing the risk of exposure to pesticide residues for workers.

In contrast, the other options do not serve this purpose. Water retention basins and wheel-move sprinklers help manage water for agriculture but do not specifically reduce pesticide handling risks. Global positioning units may assist in application precision but do not control exposure to pesticides. The focus of engineering controls is primarily to create a safer working environment by isolating hazardous processes, which closed mixing systems effectively accomplish within the context of pesticide handling.