Which one of the following is involved in the closing of stomata?

Explanation

The correct answer is Abscisic acid. Abscisic acid (ABA) is a plant hormone that plays a critical role in regulating the closure of stomata, particularly in response to drought stress. When plants experience water deficiency, ABA is synthesized in the leaves, leading to the closure of stomata to minimize water loss. In contrast, Gibberellins and Cytokinins are primarily involved in growth and developmental processes, while Ethylene is associated with fruit ripening and stress responses, but not specifically with stomatal closure.