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

The correct answer is leaf surface area. This factor plays a significant role in how effectively a plant can absorb herbicides. A larger leaf surface area can increase the amount of herbicide that comes into contact with the plant, enhancing the potential for absorption. Herbicides are often designed to target specific physiological processes in plants, and effective uptake is crucial for the herbicide to function properly.

In addition to leaf surface area, root depth, flowering stage, and fruit production also influence a plant's overall health and growth but do not directly relate to the susceptibility of a plant to herbicide absorption or action in the same way that leaf surface area does. For example, while root depth can affect the plant's access to nutrients and water, it is not a direct factor in herbicide deposition on the plant itself. Similarly, flowering stage and fruit production pertain more to plant reproductive cycles and may not influence herbicidal effects directly.