Final answer:
Children under four years old and weighing less than 40 pounds must ride in a child safety seat to ensure their protection while traveling by vehicle.
Step-by-step explanation:
Children under age four who weigh less than 40 pounds must ride in an appropriate child safety seat. This is a measure to ensure child protection in vehicles, as the anatomy of children ages 0 to 2.5 and children ages 2.5 to 6 can be quite vulnerable in the event of a car accident. It is crucial to follow these regulations to prevent injury and provide the safest travel environment for young children.