Which of the following best defines a pesticide?

A pesticide is fundamentally defined as a substance that is applied to control pests, which can include insects, weeds, fungi, and other organisms that can harm crops or human health. This definition encompasses a wide range of chemical or biological products that serve a specific purpose in pest management. By targeting unwanted organisms, pesticides play a critical role in agriculture, horticulture, and public health, helping to protect plants and improve yield.

The other choices do not accurately capture the complete essence of what a pesticide is. A type of herbicide specifically pertains to chemicals designed to target plant pests, and while they are a subset of pesticides, they do not reflect the entire category. A natural insect-repelling plant describes a biological pest control method but does not classify the plant itself as a pesticide. Finally, an environmental factor affecting crop yield refers to broader agricultural contexts that may influence growth and production but does not pertain to pest control or the definition of pesticides. Thus, the definition that best encapsulates the purpose and function of pesticides is the one stating they are substances applied to control pests.