Which of the following best describes an adjuvant?

An adjuvant is defined as a substance added to a pesticide formulation to enhance its effectiveness or improve its performance. Specifically, it serves various functions, such as improving the ability of the pesticide to adhere to the target surface, increasing the spreading or wetting characteristics, or enhancing uptake by the plant. Therefore, the choice that describes an adjuvant as a chemical that helps pesticides adhere to leaf surfaces accurately reflects its role in pesticide application, making it the correct answer.

The other options do not accurately capture the function of an adjuvant. A toxic substance that requires special handling relates more to the safety and risk management of certain chemicals and does not pertain to the supportive role of adjuvants in enhancing pesticide efficacy. A type of fertilizer used alongside pesticides implies a different product category and purpose, as fertilizers primarily provide nutrients to plants rather than enhance pesticide performance. Finally, a method to increase soil fertility refers to practices or products aimed at improving soil health and nutrient content, which is distinct from the role of an adjuvant in pesticide applications.