What is the effect on a truck-trailer combination when the parking brake in the towing vehicle is engaged (not in the test position)?

The selected combination is correct because when the parking brake in the towing vehicle of a truck-trailer combination is engaged, it ensures that the towing vehicle is mechanically braked. This is crucial for safety when the vehicle is stationary, preventing any unintended rolling.

Simultaneously, the trailer is typically braked through the service brake system using compressed air. This design is mandated by German traffic regulations, which require that trailers can be safely secured and operated, especially in terms of braking and stability.

Understanding this combination is important, as it emphasizes the need for proper braking mechanisms in different components of a truck-trailer setup, ensuring overall safety on the road.