Final answer:
Oxytocin is the hormone responsible for the release of milk from the breast, prompted by the suckling of a baby which activates nerve signals that lead to hormone release.
Step-by-step explanation:
The hormone that stimulates the release of milk from the breast is Oxytocin. While prolactin is responsible for the production of milk in the mammary glands, oxytocin is the hormone that actually triggers the milk ejection or let-down reflex. When a baby begins suckling at the breast, nerve signals are sent to the brain, which then prompts the release of oxytocin from the posterior pituitary gland. This hormone leads to the contraction of smooth muscle cells around the milk-secreting glands and ducts, pushing milk out of the breasts.