The correct answer is choice D. To stabilize the lower uterine segment. Palpating the uterus after delivery helps to determine its size, firmness and rate of descent, which are indicators of its involution. The nurse places one hand just above the symphysis pubis to support the lower uterine segment and prevent it from being pushed down by the pressure of the other hand on the fundus. This prevents complications such as uterine inversion or prolapse.

Choice A is wrong because uterine prolapse is not prevented by placing one hand above the symphysis pubis, but by supporting the lower uterine segment with that hand.

Choice B is wrong because uterine hemorrhage is not prevented by placing one hand above the symphysis pubis, but by massaging the fundus to make it firm and contract the blood vessels.

Choice C is wrong because uterine inversion is not prevented by placing one hand above the symphysis pubis, but by stabilizing the lower uterine segment with that hand and avoiding excessive traction on the umbilical cord.