Which of the following best describes an herbicide's purpose?

An herbicide is specifically designed to target and eliminate unwanted plant species, commonly referred to as weeds. The primary function of an herbicide is to disrupt the growth of plants that interfere with agricultural practices, landscapes, or gardens. By inhibiting or killing these unwanted plants, herbicides help to enhance the growth and yield of desired crops by reducing competition for resources like light, water, and nutrients.

The other options do not align with the primary purpose of herbicides. For instance, attracting beneficial insects pertains to practices that promote biodiversity and natural pest control, which is not the function of herbicides. Similarly, acting as an insect repellent and improving soil quality are separate objectives that involve different types of products and strategies in agricultural and horticultural practices. Thus, the correct description of an herbicide's purpose is its ability to kill or inhibit unwanted plant species.