Which of the following best defines a pesticide?

A pesticide is best defined as a substance intended to prevent, destroy, or repel any pest. This definition encompasses a wide range of products specifically formulated to manage and control pests such as insects, weeds, fungi, and rodents that can harm crops, livestock, or human health. The primary function of pesticides is to mitigate the negative impact that these pests can have on agricultural practices, ensuring higher yields and better quality produce.

The other options reference different agricultural or environmental practices but do not align with the core definition of what constitutes a pesticide. Enhancing crop yield, improving soil quality, and cleaning equipment are important aspects of agriculture, but they do not involve the specific role of pesticides. Thus, option C most accurately reflects the intended purpose and functionality of pesticides in pest management.