(a) oxygen
(b) temperature
(c) water in guard cells
(d) concentration of CO2 in stomata
The answer is (c) water in guard cells
Explanation: The introduction of water into guard cells aids in the opening of guard cells. The guard cell becomes turgid as a result of this. Water leaking from guard cells aids in the closing of guard cells. The guard cells will become flaccid as a result of this.