Which basic code of conduct must you observe in a tunnel?

Your answer is correct. In German traffic rules for tunnels, it is important to drive with dipped headlights even during the day to ensure good visibility and to be visible to other drivers. U-turns are prohibited in tunnels because turning back can cause dangerous traffic situations in the enclosed space. Lastly, you must follow instructions and information from tunnel personnel as they ensure safe traffic flow and handle emergencies. Observing these rules helps prevent accidents and maintain safety within tunnels.