A contact herbicide is unique in that it:

A contact herbicide is specifically designed to act on the plant tissue it directly contacts, meaning that it does not move or translocate through the plant to affect other parts. This characteristic is what sets contact herbicides apart from systemic herbicides, which are absorbed and transported throughout the plant to target growth processes in various tissues, including the roots.

For a contact herbicide to be effective, it must be applied to the surface of the plant where it can directly impact the plant cells. The lack of movement within the plant means that thorough coverage of the target area is essential for the herbicide to be effective, as it relies on contact with the specific plant tissues intended for control.

The other options suggest different modes of action or characteristics that do not apply to contact herbicides. Systemic herbicides would penetrate the leaves and translocate through the plant to affect its growth, while water activation is not a defining trait of contact herbicides, and their mode of action does not focus solely on the roots, but rather on the parts of the plant it directly contacts.