Dormancy is defined as the condition of seed when it fails to germinate even though the envoirnment condtions, usually considered favourable for active growth, are present.
A. Ethylene serves as a hormone in plants. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves.
B. Cytokinins are plant hormones that cause increased celldivision by stimulating the process of mitosis.
C. Abscisic acid owes its names to its role in the abscission of plant leaves. In preparation for winter, ABA is produced in terminal buds. This slows plant growth and directs leaf primordia to develop scales to protect the dormant buds during the cold season.
So, the correct answer is 'Abscisic acid'.