What must all motor vehicles equipped with brakes have regarding their braking system?

Answer: B — Brakes operated by one control device

All motor vehicles equipped with brakes must have a braking system that is operated by one control device. This requirement ensures that the driver can apply the brakes in a straightforward and efficient manner, enhancing safety and control while operating the vehicle. Having a singular control device allows for a more intuitive response during braking situations, which is crucial in emergency scenarios. When brakes are linked to one control, it simplifies the act of braking, allowing the driver to quickly and effectively stop the vehicle. This is a fundamental design feature in modern vehicles and contributes to overall road safety. Additionally, having multiple devices or systems that operate brakes can lead to confusion and potential failure if the driver cannot manage them effectively. Regarding the other options, while there may be certain systems that allow for manual adjustments, not all vehicles require this feature. A braking system that activates only when in motion would be ineffective and unsafe, as it would not be able to stop the vehicle when needed, and having brakes operated by different devices could complicate the driving process rather than streamline it.