Which of the following is not a plant hormone?

Concept: Plant hormones (phytohormones) are a small set of five classical growth regulators — auxins, gibberellins, cytokinins, abscisic acid, and ethylene — that plants synthesize in trace amounts to control processes such as growth, ripening, dormancy, and stress response.

Application: Among the four options, ethylene, abscisic acid, and cytokinin each belong to this set of five classical plant hormones. Vasopressin, by contrast, is a nonapeptide hormone synthesized by the mammalian hypothalamus and released from the posterior pituitary — it plays no role in plant physiology.

Cross-check: Checking each option against its true classification confirms this:

• Ethylene — a gaseous plant hormone that triggers fruit ripening, senescence, and organ abscission.

• Abscisic acid — a plant stress hormone that closes stomata and maintains seed and bud dormancy.

• Cytokinin — a plant hormone that promotes cell division and delays leaf senescence.

• Vasopressin — a mammalian hormone that regulates water reabsorption in the kidney; it is not produced by plants.

Since ethylene, abscisic acid, and cytokinin are plant hormones while vasopressin is an animal hormone, vasopressin is the one that is not a plant hormone. This is confirmed by RSSB’s official Patwar 2025 Shift 2 answer key.