California pesticide regulations mandate a closed mixing system for which type of pesticide?

The correct choice highlights California's stringent safety regulations regarding pesticides, particularly those that pose the highest risk to human health and safety. A closed mixing system is mandated for liquid pesticides that carry the label signal word "Danger" or include both "Danger" and "Poison" with the skull and crossbones symbol.

These classifications indicate that the pesticide is highly hazardous, requiring more stringent handling and mixing procedures to minimize the risk of exposure to applicators and the environment. A closed mixing system helps to prevent spills and accidental releases of these toxic substances, ensuring that they are handled safely. This is essential not only for the safety of the individuals applying the pesticide but also for protecting surrounding ecosystems and communities.

Other labels such as "Caution" or "Warning" signify lower levels of toxicity and therefore do not necessitate the heightened precautions associated with a closed mixing system.