What describes a "pesticide"?

A pesticide is specifically defined as a substance used for controlling pests, which includes insects, weeds, fungi, and other organisms that are considered undesirable or harmful in agricultural, residential, or public health contexts. This definition encompasses a wide range of products, including insecticides for targeting insects, herbicides for weed control, and fungicides for managing fungal diseases.

The other options, while they may relate to pest management or plant care in certain contexts, do not capture the full breadth of what a pesticide is. A natural insect repellant is a type of pesticide but does not account for the many other forms pesticides can take. A fertilizer for plants is meant to promote growth rather than control pests, and an herb used in cooking is intended for culinary purposes, not for pest management. Therefore, identifying a pesticide as a substance specifically used for controlling pests is the most accurate and comprehensive definition.