What is the definition of herbicide?

A herbicide is specifically defined as a chemical substance designed to kill or inhibit the growth of unwanted plants, commonly referred to as weeds. This definition underscores the primary function of herbicides in agriculture, landscaping, and other contexts where the control of plant growth is necessary for the management of crops or the maintenance of certain areas. Unlike substances that enhance growth or improve soil quality, herbicides are aimed explicitly at the elimination of competing plant species, making option C the most accurate choice.

In contrast, the other options either refer to unrelated functions or lack the specificity needed to define a herbicide. For instance, options regarding enhancement of growth or soil improvement do not pertain to herbicides, which focus solely on plant suppression rather than promotion. Additionally, although plant regulation terms may encompass a broader range of activities, they do not capture the specific action of herbicides defined by their chemical purpose in controlling unwanted vegetation.