The correct answer is: A. 1 in 400 or steeper.
Hand shunting of vehicles in gradients 1 in 400 or steeper is totally prohibited. This is because it is very dangerous and can lead to accidents. When a vehicle is shunted up a gradient, the weight of the vehicle pushes down on the wheels, which can cause them to slip. This can make the vehicle lose control and roll over. In addition, if the vehicle is shunted down a gradient, the weight of the vehicle pushes it forward, which can cause it to accelerate out of control. This can also lead to accidents.
For these reasons, it is important to avoid hand shunting vehicles in gradients 1 in 400 or steeper. If you must shunt a vehicle in a gradient, make sure that you take all necessary safety precautions, such as using a wheel chock to prevent the vehicle from rolling.