Which one of the following is a plant hormone?

Correct answer: D (Cytokinin). Cytokinins are one of the five classical plant growth regulators, along with auxins, gibberellins, abscisic acid and ethylene. They promote cell division (cytokinesis), which is why they were first identified in the 1950s from coconut milk and kinetin. They also delay leaf senescence and, along with auxin, control shoot and root differentiation in tissue culture.

• A (Insulin): A polypeptide secreted by the beta cells of the pancreas. It regulates blood glucose in animals, so it is an animal hormone.
• B (Thyroxin): Produced by the thyroid gland. It regulates metabolic rate in vertebrates and is not found in plant hormone systems.
• C (Oestrogen): A steroid hormone produced mainly by the ovaries and placenta. It controls female reproductive functions.
• E (None/More than one): Wrong, because D is a plant hormone.

Core concept: phytohormones act in very low concentrations and coordinate growth, development and stress responses. For Mains, cytokinins matter in plant tissue culture, post-harvest preservation of vegetables and the auxin-cytokinin ratio that controls organogenesis.

Trap: Students see the word 'hormone' and pick the most familiar one, or assume that because several options are hormones, E must be right. Check the source organism for each option first.