What must you do before detaching a semitrailer?

The selected combination is correct because all three steps are essential for safely detaching a semitrailer according to German traffic regulations.

1. Locking the wheels of the semitrailer and extending the supporting legs prevents it from rolling away or tipping over after disconnection, which ensures safety in the surrounding area.

2. Disconnecting all hose and cable connectors ensures that all power and air lines are safely detached, preventing potential hazards such as leakage or electrical malfunctions.

3. Releasing the coupling lock and opening the hitching device of the fifth wheel is necessary to physically detach the semitrailer from the truck tractor, making it clear that all connections have been safely removed.

Each step plays a crucial role in the overall safety of the detachment process, guarding against accidents and ensuring compliance with traffic laws. Following these steps properly can help avoid damage to the vehicles and any surrounding infrastructure, as well as improve safety for other road users.