Final answer:
The Moro reflex is the correct answer to the question of which reflex is exhibited when newborns are held horizontally on their stomachs and stretch out their arms and legs. It's an instinctive behavior present from birth to about three or four months of age.
Step-by-step explanation:
When newborns are held horizontally on their stomachs, they stretch out their arms and legs in what is known as the Moro reflex. This is a type of instinctive behavior that is normally present in infants from birth until about three or four months of age. It occurs in response to a sudden loss of support when the infant feels as though it is falling.
The Moro reflex consists of three distinct components: the baby suddenly spreading out the arms, bringing the arms back in toward the body, and usually crying. This reflex may serve the purpose of helping the infant reach out and grab onto a caregiver, potentially protecting them from harm if they were to fall.