- A Calcium
- B Iron
- C Potassium
- D Magnesium
Correct answer: C. Potassium
Explanation: Potassium (K+) ions actively accumulate in guard cells in the light, increasing osmotic pressure. Water enters by osmosis, making guard cells turgid and opening stomata.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.