Seatbelts, head restraints, and door locks are considered part of the vehicle's:

Seatbelts, head restraints, and door locks are considered part of the vehicle's protective system because they are specifically designed to enhance the safety and security of the vehicle's occupants. The protective system encompasses all features that help minimize injury in the event of an accident and secure the vehicle against theft or unauthorized access. Seatbelts restrain occupants during a collision, head restraints prevent whiplash injuries, and door locks protect against unauthorized entry.

Other systems like the engine, suspension, or electrical systems serve different purposes, such as powering the vehicle, supporting its weight, and controlling electronic functions, but they do not focus primarily on safety and security. Thus, categorizing these safety features under the protective system emphasizes their role in safeguarding both the driver and passengers while driving.