Children under the age of ___ must be in a car safety seat.

The correct answer is that children under the age of 8 must be in a car safety seat. This requirement is based on safety guidelines intended to protect young passengers who have not yet reached the size and weight thresholds for using standard seat belts safely. Children in this age group are typically still too small to be adequately restrained by adult seat belts alone, which could result in serious injury during a crash.

Car safety seats are designed to provide better protection for children by properly securing them and positioning them to ensure that seat belts fit correctly when they are older. The guiding principle behind this regulation is that as children grow, their physical development impacts the effectiveness of the car's safety features. Ensuring that children under this age are in a suitable safety seat greatly enhances their safety in vehicles, which is why many jurisdictions enforce such regulations.