Which type of herbicide is classified as a contact herbicide?

A contact herbicide is specifically designed to affect only the external tissues of a plant. This means that it works by directly contacting the plant surfaces where it is applied, causing damage to the leaves, stems, or other exposed parts. Since it does not penetrate deeply into the plant or move through its vascular system, the effects are limited to the areas where the herbicide has been applied.

The other classifications of herbicides involve different mechanisms. For instance, herbicides that are absorbed throughout the plant will be taken up by the roots or leaves and then translocated throughout the plant, affecting its overall physiology. Similarly, herbicides that move within the vascular tissue are systemic and can target specific locations in the plant that can lead to more extensive damage. Herbicides that target only root systems focus on the underground parts of the plant, which is a different mode of action altogether. Thus, understanding that contact herbicides work on the foliage without systemic activity clarifies the distinction and makes option B the appropriate choice.