Which statement best defines a herbicide?

A herbicide is defined as a chemical that inhibits plant growth or kills plants. This definition encompasses the primary function of herbicides, which is to target unwanted vegetation, including weeds and invasive plant species. They are specifically designed to disrupt the growth processes of plants, either by interfering with their metabolism, causing cell damage, or inhibiting photosynthesis.

Herbicides can vary widely in their chemical composition and mechanism of action, but their essential role is to manage or eliminate plants that may compete with desired crops or disturb ecological balances in both agricultural and non-agricultural settings.

The other options do not accurately capture the essence of what constitutes a herbicide. For example, a chemical that promotes plant growth contradicts the purpose of a herbicide, while a natural substance derived from algae does not encompass the wide range of synthetic and natural herbicides available. Additionally, a compound used exclusively for aquatic environments narrows the definition too much, as many herbicides are applicable in terrestrial settings as well.