Explanation
The correct answer is A: K ions.
The opening and closing of stomata, which are small openings on the surface of leaves and stems of plants, is regulated by the movement of ions and water across the guard cells that surround the stomata. When the concentration of K+ ions increases in the guard cells, water moves into the cells, causing them to swell and the stomata to open. Conversely, when the concentration of K+ ions decreases, water moves out of the cells, causing them to shrink and the stomata to close.
Therefore, the influx of K+ ions into the guard cells is responsible for the opening of stomata, while their efflux results in their closure. Na+ ions, Fe ions, and Mg ions are not directly involved in the regulation of stomatal movement.
