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

If the brake line hose with the yellow coupling head tears between the towing vehicle and the trailer, two important things happen according to German traffic rules for vehicles with compressed air brake systems. First, the four-way protection valve on the towing vehicle maintains the safety pressure in its own service brake system, which means the towing vehicle's brakes will still work properly. Second, the supply line to the trailer is automatically vented. This causes the trailer’s brakes to be applied, making sure the trailer comes to a safe stop in case of a disconnection or a line break. These safety mechanisms are in place to prevent runaway trailers and to make sure both vehicles remain as safe as possible if a brake line fails. That’s why both answer 1 and answer 2 are correct.