What should you do if your vehicle's brakes fail?
Answer: C — Use the emergency or parking brake to slow down or stop.
If your vehicle's brakes fail, option C is the best choice because it will allow you to control the speed and direction of your vehicle. Option A may not work in a vehicle with anti-lock brakes and could potentially cause skidding. Option B is not recommended because it can put strain on the engine and cause damage. Option D should only be used as a last resort if there are no other options available.