The correct answer is (d), Cytokinin.
Cytokinins are a group of plant hormones that promote cell division and cell expansion. They are also involved in the regulation of plant growth and development, including the regulation of leaf senescence, the promotion of root growth, and the inhibition of fruit ripening.
Insulin is a hormone that is produced by the pancreas and helps the body to use glucose for energy. Thyroxin is a hormone that is produced by the thyroid gland and helps to regulate metabolism. Estrogen is a hormone that is produced by the ovaries and helps to regulate the menstrual cycle and female sexual development.
All of these hormones are important for human health, but only cytokinin is a plant hormone.