Answer: C. Prolactin
Prolactin is a hormone that controls milk production together with oxytocin. The anterior lobe of the pituitary gland secretes prolactin and the posterior lobe secretes oxytocin when a baby suckles at the breast. The level of prolactin in the blood slowly increases during pregnancy, and stimulates the growth and development of the mammary tissue, in preparation for the production of milk after delivery.
Moreover, Prolactin is in the highest level in about 30 minutes after the beginning of the feed, which is most important for producing milk for the next feed.