Who must always wear a seat belt while riding in a vehicle?

Answer: C — All passengers in a vehicle

All passengers in a vehicle must always wear a seat belt while riding in a vehicle. This includes passengers of all ages, not just those over 15 or under 10. It is important for everyone to wear a seat belt to protect themselves and others in case of a car accident. Even though the age may vary for certain laws or regulations, it is always best to follow the general rule that all passengers in a vehicle should wear a seat belt. Therefore, options A, B, and D are incorrect.