Correct answer: (1) Abscisic acid

Explanation:

• Abscisic acid (ABA) is the key plant hormone responsible for regulating the opening and closing of stomata.

• Under water stress or drought conditions, ABA levels increase and trigger stomatal closure to reduce water loss by transpiration.

• ABA signaling activates ion channels in guard cells, causing loss of turgor and stomatal closure.

• Indole-3-acetic acid (IAA or auxin), cytokinins, and ethylene do not directly control stomatal movements like ABA.

Option details:

• (1) ABA — correct; controls stomatal closure during stress.

• (2) IAA (Auxin) — promotes growth, not stomatal control.

• (3) Cytokinin — promotes cell division and delays senescence but not stomatal movement.

• (4) Ethylene — involved in senescence and stress signaling, but not primary for stomata.

  Abscisic acid is crucial for plant water management, signaling guard cells to close stomata when environmental conditions threaten water availability, thus enhancing drought resistance and water use efficiency.