Which substance is used to enhance the effectiveness of a pesticide?

The correct choice is adjuvant because adjuvants are specifically formulated substances added to pesticides to improve their performance. They can enhance the stability, efficacy, and application characteristics of the pesticide, allowing for better adherence to target surfaces, improved penetration, or increased effectiveness against pests. This can lead to more efficient treatments and reduced amounts of pesticide needed, which is beneficial for both environmental and economic reasons.

In contrast, while fertilizers provide nutrients to plants, they do not enhance the efficacy of pesticides. Water, although essential for diluting and applying many pesticide formulations, does not inherently enhance their effectiveness beyond this role. Soil, while it serves as the medium in which plants grow and where pesticides may be applied, does not function to enhance pesticide efficacy directly; rather, it can affect pesticide behavior and degradation. Adjuvants stand out as the specific agents aimed at making pesticides work better.