Why are passengers thrown forward against their seat belts when a car stops suddenly?

The correct answer involves understanding the concept of inertia, which is the tendency of an object to resist changes in its state of motion. When a car is moving at a certain speed and suddenly comes to a stop, the passengers inside the car retain their forward momentum due to inertia.

As the vehicle decelerates quickly, the seats and the car itself come to a halt, but the passengers, who are not firmly attached to the vehicle, continue moving forward. This motion occurs until they are restrained by their seat belts. Therefore, it's the inertia of each passenger that causes them to lurch forward against the seat belt when the vehicle stops suddenly.

The other choices, such as the sudden deceleration of the car and the friction of the seat belt, relate to the dynamics of the event but do not directly explain the reason for the forward motion of the passengers. The weight of the passengers might influence the forces involved but does not specifically account for the effect of inertia in this scenario.