Final answer:
Oxytocin is correctly paired with its action of stimulating uterine contractions during childbirth, as it is secreted by the pituitary gland and its release increases the intensity of contractions through a positive feedback mechanism.
Step-by-step explanation:
The hormone correctly paired with its action from the choices provided is oxytocin, which stimulates uterine contractions during childbirth. Oxytocin is an endocrine hormone secreted by the pituitary gland. Throughout most of pregnancy, the uterus is not very sensitive to oxytocin. Towards the end of pregnancy, the number of oxytocin receptors in the uterus increases significantly, making the smooth muscle cells more sensitive to oxytocin.
As childbirth progresses, uterine contractions prompt more oxytocin to be released, which in turn increases the intensity of the contractions. This positive feedback mechanism continues until childbirth is complete. Therefore, option (A) oxytocin—stimulates uterine contractions during childbirth is the correct pairing.