The correct answer is frictional force.

When brakes are applied to a moving vehicle, the brake pads press against the rotating wheels. This contact creates friction between the brake pads and the wheels. This frictional force opposes the motion of the wheels, causing them to decelerate and eventually stop. The frictional force converts the kinetic energy of the moving vehicle into heat energy, dissipating it and bringing the vehicle to a halt.

Let's examine why the other options are incorrect:

• Gravitational force: Gravity acts downwards towards the Earth's center. While it affects the vehicle's weight, it doesn't directly cause braking.
• Van der Waals forces: These are weak intermolecular forces and are not significant enough to cause the substantial deceleration needed to stop a vehicle.
• Covalent forces: These are strong chemical bonds within molecules and have no role in the macroscopic process of braking.

Therefore, the primary force responsible for stopping a vehicle when the brakes are applied is frictional force.