Final answer:
The sentence should be completed with 'must,' indicating the legal requirement for the use of safety belts in passenger vehicles. This governmental mandate serves to protect individuals and society by reducing the likelihood and severity of injuries in vehicular accidents.
Step-by-step explanation:
The correct answer for the fill-in-the-blank question is 'must.' The sentence should read: 'Any person who operates a passenger vehicle is equipped with safety belts must use them.' This is because the use of safety belts is mandated by law in many places, and laws are put in place for the protection of both the individuals and the society at large. Failure to use a safety belt can lead to driver impairment in accidents, which carries a burden on society for potential medical and physical care.
Although should indicates a recommendation, and can and may imply possibility or permission, 'must' is the word that conveys the legal obligation for wearing safety belts. This underscores the significance of seat belt laws that aim to safeguard passengers and reduce accidents with serious injuries, ultimately requiring the community to bear the medical and financial responsibilities.