Identify the part that controls the closing and opening of the stomatal pore in leaves of plants. Select the correct option.

As given in the NCERT text: 'The opening and closing of the pore is a function of the guard cells. The guard cells swell when water flows into them, causing the stomatal pore to open. Similarly the pore closes if the guard cells shrink.'

Option C (Guard cells) — Correct. The two guard cells flank each stomatal pore. Changes in their turgor pressure (due to water flowing in or out) directly cause the pore to open or close, thereby regulating gas exchange and water loss.

Option A (Stomata) — Incorrect. The stoma is the pore itself, i.e., the structure being controlled, not the controlling agent.

Option B (Epidermal cells) — Incorrect. Ordinary epidermal cells form the leaf's outer covering but do not regulate the stomatal pore; only the specialised guard cells (a modified pair of epidermal cells) do this.

Option D (Chloroplasts) — Incorrect. Chloroplasts are cell organelles that carry out photosynthesis inside guard cells (and mesophyll cells); they are not structures that physically open or close the pore.