Which one of the following plant hormones promotes stomatal closure during drought stress?

Abscisic acid (ABA) is widely recognized as the "stress hormone" in plants. During environmental stresses such as drought, ABA levels increase significantly in the leaves. It functions by signaling the guard cells that surround the stomatal pores. ABA triggers the efflux of ions, particularly potassium (K⁺), from the guard cells, leading to a loss of turgor pressure. As the guard cells become flaccid, the stomatal pore closes, which effectively reduces water loss through transpiration and helps the plant survive water-deficient conditions.

In contrast, Auxins, Gibberellins, and Cytokinins are primarily growth-promoting hormones involved in cell elongation, seed germination, and cell division, respectively, and do not typically mediate the stomatal closure response to drought.