Final answer:
c. Oxytocin
The hormone necessary for milk ejection, also known as the "let-down" reflex, is oxytocin. It causes contractions in the mammary glands that push milk into the ducts when an infant suckles.
Step-by-step explanation:
The hormone necessary for the "let-down" or milk ejection reflex is oxytocin. This reflex is a neuroendocrine response triggered when an infant suckles at the breast. Sensory nerve fibers in the areola are stimulated, sending signals to the hypothalamus, which prompts the posterior pituitary to release oxytocin. Once released, oxytocin causes the myoepithelial cells in the mammary glands to contract, pushing milk from the alveoli into the lactiferous ducts and sinuses, and ultimately through the nipple pores to the infant. This process is known as the let-down reflex and it usually occurs within one minute of the infant beginning to suckle.