Which of the following is defined as a chemical that kills or repels animal or insect pests?

A pesticide is defined as a chemical that is used to kill, repel, or control a variety of pests, including insects, animals, weeds, fungi, and other organisms that can be harmful to crops, humans, or the environment. The term 'pesticide' encompasses several types of chemicals based on the specific pest they target, which is why it is the broadest term in this context.

Fertilizers are substances that provide essential nutrients to plants to promote growth but do not necessarily kill or repel pests. Herbicides specifically target and kill weeds, which are unwanted plants, while insecticides are formulated to control insect populations. While insecticides fall under the umbrella of pesticides, the broader category captures a wider range of pest control agents. Thus, the correct answer points to the general classification of chemicals that serve the pest control function.