What will be the effect if the brake line hose with the yellow coupling head tears between the towing vehicle and trailer during braking?

The combination of answers you selected is correct because it aligns with the regulations around towing vehicles and trailers in German traffic law.

When the brake line hose with the yellow coupling head tears, the four-way protection valve ensures that the towing vehicle maintains safety pressure in its service brake system, preventing a total loss of braking power. This is a crucial safety feature meant to protect the braking capabilities of the towing vehicle. Meanwhile, the supply line is automatically vented, which means that the trailer's braking system will engage to stop the trailer. This action ensures that the trailer does not become a hazard on the road.

Missing out on any part of this combination could lead to unsafe situations, such as the trailer being unable to brake properly or the towing vehicle losing significant braking control, which is why it is essential to understand the entire mechanical process involved. This not only aids in safety but also helps in complying with traffic regulations in Germany.