What is the primary method to stop a vehicle equipped with air brakes during an emergency?

Answer: B — Apply controlled braking and downshift

The primary method to stop a vehicle equipped with air brakes during an emergency is to apply controlled braking and downshift. This approach is crucial for maintaining control over the vehicle during a sudden stop. Controlled braking allows the driver to gradually reduce speed while ensuring that the brakes are not applied so abruptly that they lock up, which could lead to a loss of traction and control. In addition to braking, downshifting helps in slowing down the vehicle by using the engine's resistance, thereby assisting the braking system and enhancing the overall stopping process. While other options may seem relevant, they do not provide the same level of safety or effectiveness. Using emergency signals does not aid in stopping the vehicle, and turning off the engine takes away the driver’s ability to control the vehicle effectively. Slamming on the brakes can cause the wheels to lock, leading to potential skidding and loss of control instead of a safe stop. Therefore, combining controlled braking with downshifting is the safest and most effective method to stop a vehicle equipped with air brakes in an emergency situation.