Final answer:
The reflex described where an infant reacts to the sensation of falling by spreading out and then pulling in their arms is called the Moro reflex.
Step-by-step explanation:
The primitive reflex you're describing, where an infant is held in a supine position and the head is dropped slightly, causing the arms to abduct and extend followed by flexion, is known as the Moro reflex. This reflex is an instinctual response present from birth until about three to four months of age. When babies feel as though they are falling, this reflex kicks in, involving a sudden spreading out of the arms, pulling them back in toward the body, and typically, crying. This response could help an infant reach out and grab onto a caregiver if they were actually falling. The Moro reflex, along with the grasping reflex, where a baby automatically grips anything that touches the palms, are survival mechanisms that help infants maintain close physical contact with their caregiver.