Which of the following is a morphological factor affecting plant susceptibility to herbicides?

Leaf surface area is a key morphological factor affecting plant susceptibility to herbicides because it significantly influences the amount of herbicide that can be absorbed by the plant. Larger leaf surfaces can enhance the absorption rate of contact herbicides, as they provide a greater area for the chemical to adhere to and penetrate. This increased absorption can lead to more effective control of the plant.

In contrast, while root depth and the flowering stage can certainly influence a plant's overall health and life cycle, they are not direct morphological factors that contribute to the herbicide's uptake. Additionally, fruit production impacts the reproductive success of the plant but does not directly relate to its susceptibility to herbicides in the context of chemical absorption and action.