You are driving a laden motor vehicle equipped with a manual gearbox. On a steep descent your speed is steadily increasing even though the permanent brake is activated. What should you do?

When driving down a steep descent with a laden vehicle, it is crucial to control your speed effectively to prevent potential accidents. The choice to brake using the service brake and select a lower gear is correct because:

1. **Braking**: The service brake helps to slow down the vehicle when needed. It is essential to use it in conjunction with gearing down to manage your speed effectively on a steep incline.

2. **Selecting a Lower Gear**: Shifting to a lower gear increases engine braking. This is important as it allows the engine to assist in controlling the vehicle's speed, thus reducing reliance on the brakes alone and preventing brake fade due to overheating.

Not choosing a higher gear, as suggested in one of the incorrect options, would increase the vehicle's speed further, making the descent more dangerous. Similarly, deactivating the permanent brake while solely relying on the service brake disregards the importance of two methods working in conjunction to maintain speed control.

Following these steps is consistent with the principles in German traffic regulations regarding safe driving practices, which emphasize speed control and ensuring proper vehicle handling, particularly in challenging driving conditions.