The opening and closing of stomata is regulated by:

As the NCERT chapter states: "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." Thus the opening and closing of stomata is regulated by the amount of water in the guard cells (option D). Option A is incorrect because $\mathrm{CO}_2$ concentration influences whether the plant needs to open stomata for gas exchange, but the actual mechanism of opening/closing is the turgidity change in guard cells caused by water movement, not $\mathrm{CO}_2$ acting directly on the pore. Option B is incorrect because temperature is not identified in the chapter as the regulating factor of stomatal movement. Option C is incorrect for the same reason as A — $\mathrm{O}_2$ concentration is not the mechanism described; it is water flow into/out of guard cells that swells or shrinks them and thereby opens or closes the pore.