Answer:
Abscisic acid
Step-by-step explanation:
Abscisic acid (ABA) is a plant hormone, which is produced by plants during stress conditions, such as drought (water-deficit conditions), thus, also known as stress hormone.
In dry conditions, it reduces water loss from plant leaves by reducing the rate of transpiration. ABA causes alteration in the osmotic pressure of stomatal guard cells of leaves. It leads to shrinking and closing of stomata, due to which transpiration is reduced.
Thus, the correct answer is 'abscisic acid.'