Final answer:
The primary safety precaution for vehicle occupants is buckling their seat belts, which is crucial for minimizing the risk of death or serious injury during a collision.
Step-by-step explanation:
The safety precaution that should be ensured for all vehicle occupants is c) Buckle their seat belts. Ensuring that every person in the vehicle is buckled up is essential for their safety. Like wearing a safety helmet for biking and skating, or using hearing protectors in loud environments, seat belts protect drivers and passengers during a vehicle operation. It's not just about preventing ejections from the vehicle but also minimizing the impact within the car during a possible collision. This simple yet crucial act can reduce the risk of death or serious injury significantly.