What causes a vehicle to skid on the roadway?

1. Quick wear of the clutch and of the gearbox.
2. Correct answer: When the braking forces or the vehicle's inertia exceed the tire’s ability to grip of the roadway.
3. Excessive change of gears.
4. When the engine is worn out and cannot stabilize the vehicle.

WhyA skid starts when braking or inertia forces exceed the tires' grip on the road. Engine or gearbox wear does not cause it.