The correct option is
A Parturition
Oxytocin is a hormone which is released from the pituitary gland. It is responsible for the muscular contraction which occurs during the parturition. The fully developed baby exerts a pressure on the walls of the uterus and as a result, it signals the pituitary gland to release oxytocin. The muscular contractions are responsible for the expulsion of the baby out of the uterus.
So, the correct option is 'Parturition'.