The correct answer is inertia. Inertia is the tendency of an object to resist changes in its motion, whether that is to stay at rest or to keep moving in a straight line at a constant speed. When you hit the brakes while driving, your body wants to keep moving forward due to its inertia. However, the seatbelt applies a restraining force and stops your body from continuing to move forward, preventing injury. This is why you feel your body moving forward before the seatbelt stops you.
