Final answer:
An infant's sucking is a reflex behavior that is instinctive and vital for the infant's survival, through which they naturally and efficiently gain nourishment.
Step-by-step explanation:
An infant sucking is a good example of a reflex behavior and is also an instinct. Newborns have a sucking reflex that is an instinctive behavior necessary for survival. This reflex is an automatic, unlearned, sucking motion utilized by infants to intake nourishment, and it is present from birth. It's a part of a set of newborn reflexes which also include the grasp reflex and the rooting reflex. These reflexes help the infant to find and latch onto the mother's nipple or a bottle's nipple and are critical for the infant's early nutrition and bonding.