Final answer:
Children under 2 years old must be restrained with proper child-restraint equipment, consistent with safety recommendations and laws that aim to protect infants and toddlers in vehicles.
Step-by-step explanation:
The question pertains to the safety regulations concerning the use of child-restraint equipment for children when they are passengers in a vehicle. While the regulations can vary by country and state, a common rule is that children under a certain age must be restrained properly. This typically includes infants and toddlers.
If we are adhering to a standard that is commonly recommended, children under 2 years old must be restrained in a rear-facing car seat to reduce the risk of serious injury in the event of an accident. Therefore, option (b) Under 2 years old would be the most common and generally recommended answer.