The correct option is A Potassium
Potassium is an important macronutrient for plants. Potassium ions cause turgor movement of guard cells. The gain of potassium ions by guard cells causes a decrease in water potential of guard cells, which leads to inflow of water into the guard cells. This makes guard cells turgid and they swell. The increase in turgidity of guard cells opens the stomatal pore. Movement of potassium ions out from guard cells results in an increase in water potential, they lose water due to exosmosis, become flaccid and stomatal pore closes.