Potassium is essential for plants primarily because it:

Potassium plays a crucial role in enhancing crop resistance to disease, making it an essential nutrient for plants. It helps in the regulation of various physiological processes within the plant, including osmotic balance and enzymatic functions, which are vital for growth and overall health. By strengthening the plant's immune response and helping in the synthesis of proteins that fight off pathogens, potassium increases the resilience of crops against diseases.

While the other options also relate to important attributes of plant health, they do not illustrate the primary role of potassium as effectively as aiding disease resistance. Improving root structure is more directly linked to other nutrients such as phosphorus. Enhancing the optical properties of leaves generally pertains to other factors, including light exposure and chlorophyll content rather than potassium itself. Similarly, while potassium does have a role in enzyme production, it is not as directly associated with the stimulation of photosynthetic enzyme production as other nutrients might be. Thus, the focus on potassium's role in disease resistance represents its most significant and direct impact on plant health.