What is defined as a substance used to destroy, repel, or prevent pests?

The definition provided in the question aligns precisely with that of a pesticide. Pesticides are substances specifically designed to destroy, repel, or prevent various types of pests, which can include insects, weeds, fungi, and rodents. The broad nature of this term encompasses various formulations tailored for specific pest issues, making it a key tool in pest management strategies.

Fertilizers are primarily used to enhance plant growth and provide essential nutrients, rather than to control pests. Herbicides are a specific category of pesticides that target unwanted plants or weeds, indicating that they do not encompass the entire spectrum of pest control. Insect repellents are designed to deter insects rather than destroy or prevent them, further illustrating the distinction between these terms. Thus, while all the other options relate to aspects of plant cultivation or pest management, only pesticide encompasses the full range of substances focused on pest control, making it the most accurate answer.