What must a driver do before leaving a vehicle parked on a grade?

Answer: B — Turn the front wheels so the vehicle rolls off the road if it moves

Before leaving a vehicle parked on a grade, the driver must turn the front wheels in the direction of the curb or edge of the road to prevent the vehicle from rolling into traffic. This is important because if the vehicle does not have this safety measure, it could potentially cause an accident. It is also important to not just engage the parking brake, as this alone may not secure the vehicle on a steep incline. Additionally, leaving the vehicle in neutral is not recommended as it could still allow the vehicle to roll. Locking the doors is also not an adequate safety measure in this situation as it does not prevent the vehicle from rolling. Therefore, option B is the correct answer as it is the safest and most appropriate action to take before leaving a parked vehicle on a grade.