Explanation:

The correct option is (A) Potassium. Potassium ($\text{K}^+$) ions are crucial for regulating the turgor pressure within guard cells, which directly controls the opening and closing of stomata. When $\text{K}^+$ ions accumulate in guard cells, water enters by osmosis, increasing turgor and opening the stomata. Conversely, $\text{K}^+$ efflux leads to water loss and stomatal closure. Option (B) Phosphorus is vital for energy transfer and nucleic acids but not stomatal movement. Option (C) Nitrogen is essential for proteins and chlorophyll but not direct stomatal control. Option (D) Zinc is a micronutrient for enzyme function, not stomatal mechanics.