Herbicides are primarily used to:

Herbicides are specifically designed to target and destroy unwanted vegetation, which includes weeds and other plants that may compete with desired crops or landscaping. These chemicals work by interfering with the normal growth processes of plants, leading to their death.

The application of herbicides is a common practice in agricultural settings, as well as in residential and commercial landscaping, to maintain the health and aesthetic appeal of desired plants by managing the presence of competing plant species. This makes it clear why the function of herbicides is directly tied to their ability to eliminate unwanted plant life.

The other options, while related to pest control, do not correctly describe the primary function of herbicides. For example, repellents are used against insects, which is not the role of herbicides. Similarly, controlling gastropods (like snails and slugs) and eliminating fungus are tasks associated with different types of pest control products rather than herbicides. Thus, the focus of herbicides is strictly on vegetative management, validating the selection of the answer regarding their main usage.