Final answer:
Aluminum foil rolls are carried on ambulances primarily to keep newborns warm, preventing hypothermia by reflecting their body heat back towards them.
Step-by-step explanation:
The purpose of carrying an aluminum foil roll on an ambulance is A. To keep a newborn warm. By wrapping a newborn in aluminum foil, rescue personnel can reduce the loss of body heat through radiation, conduction, and evaporation. This is crucial because newborns, especially preterm infants, have a higher risk of hypothermia due to their still-developing body temperature regulation mechanisms. The shiny surface of the foil reflects the baby's body heat back towards them, effectively keeping them warmer than they would be otherwise. Aluminum foil can also serve in a similar capacity to protect amputated parts by insulating them during transport to a medical facility; however, this is not the primary purpose mentioned in the question.