. Stomata open and close due to
(a) presence of valves.
(b) hormonal control.
(c) turgor pressure of guard cells.
(d) concentration gradient of the gases.
(d) turgor pressure of guard cells.
Explanation. Opening and closing of stomata is guarded by the guard cells. Stomata open when guard cells are turgid (full of water) and closes when guard cells are flaccid (loss of water).