What type of herbicide requires contact application to be effective?

Contact herbicides are specifically designed to work upon direct application to the plant surface. They require the plant to come into contact with the chemical for it to be effective in controlling weeds or unwanted vegetation. These herbicides act rapidly by disrupting processes such as cell division or photosynthesis in the plant tissues they contact, leading to the plant's death.

In contrast, systemic herbicides are absorbed by the plant and translocated to various tissues, allowing them to affect the entire plant even if not all parts are directly treated. Pre-emergent herbicides, on the other hand, prevent the germination of seeds by creating a chemical barrier in the soil rather than relying on contact with existing plant material. Atrazine is a specific type of herbicide that functions primarily as a systemic herbicide, targeting certain types of weeds, and does not require contact application for effectiveness. Thus, the correct identification of contact herbicides reveals their unique mode of action distinct from the other types of herbicides listed.