The correct option is Abscisic acid.
Explanation
Phytohormones (plant hormones) are chemical regulators produced naturally in plants. They are broadly classified into two categories based on their physiological effects: Plant Growth Promoters (e.g., Auxins, Gibberellins, Cytokinins) and Plant Growth Inhibitors (e.g., Abscisic Acid, Ethylene).
Analysis of Options:
- Abscisic Acid (ABA): This hormone acts primarily as a growth inhibitor. It plays a critical role in the abscission of leaves and fruits and the induction of dormancy in seeds and buds. By inducing dormancy, it helps seeds withstand desiccation and other unfavorable growth factors. It is also known as the "stress hormone" because it triggers stomatal closure during water stress.
- Cytokinins: These are growth promoters that primarily stimulate cell division (cytokinesis). They help produce new leaves and chloroplasts and delay leaf senescence.
- Gibberellins: These are growth promoters responsible for stem elongation (bolting), fruit growth, and breaking seed dormancy (antagonistic to ABA).
- Auxin: This hormone promotes cell elongation, root formation, and apical dominance. It is essential for growth rather than inhibition.
Key Takeaway: Abscisic acid is the specific plant hormone responsible for inhibiting growth and inducing dormancy, functioning as an antagonist to growth-promoting hormones like Gibberellins.