What must you do to prevent a two-axle bus from rolling down a slope?

Your answer is correct because to prevent a two-axle bus from rolling down a slope, German traffic rules require both engaging the parking brake and placing a wheel chock under a rear wheel. The parking brake secures the vehicle overall, but on a slope, there is still a risk that the bus might move due to gravitational forces. Using a wheel chock provides an additional physical barrier to prevent rolling. The bus-stop brake is not intended for parking or securing the vehicle but for temporarily stopping the bus at stops, so it is not enough to prevent rolling on a slope. In summary, combining the parking brake with a wheel chock ensures maximum safety on inclines.