Final answer:
The correct child restraint system for an infant under 1 year old and less than 20 pounds is a rear-facing car seat, ensuring safety for the youngest children.
Step-by-step explanation:
The required child restraint system for an infant under the age of 1 and weighing less than 20 pounds is a rear-facing car seat. It is critically important for the safety of children, particularly those in the age group of children ages 0 to 2.5, to be placed in a car seat that is appropriate for their age, weight, and height. Rear-facing car seats are designed to support the head, neck, and spine of infants in the event of a sudden stop or collision.