Final answer:
Uterine contractions in the fourth stage of labor aim to control bleeding by helping to expel the placenta and compress blood vessels to prevent excessive blood loss.
Step-by-step explanation:
The purpose of uterine contractions in the fourth stage of labor is to control bleeding (option a). After the baby is born, the uterus continues to contract to help expel the placenta and compress the blood vessels in the area where the placenta was attached. This helps to prevent excessive bleeding and is the body's natural way of minimizing blood loss post-delivery.