Final answer:
Children must ride in an approved child safety seat or booster until they are generally 8 years (option a) old, at which point most can transition to using the vehicle's regular seat belts.
Step-by-step explanation:
The question refers to the regulations surrounding child safety seats and at what age children are required to use them. In general, these regulations can vary by country or even by state or region within a country. However, a widely accepted guideline is that children ages 0 to 2.5 must always ride in a rear-facing car seat, children ages 2.5 to 6 should use a forward-facing child seat, and children ages 6-13 should be in a booster seat until they are big enough for the vehicle's seat belt to fit properly, usually when they have reached 4 feet 9 inches in height and are between 8 and 12 years old.
Option (a) states age 8, which is most commonly correct for when a child can transition out of a booster seat and use a regular seat belt. Each of these age groups marks critical stages in development where specific types of car seats and restraints provide the appropriate safety measures.