How are braking systems on combination vehicles usually controlled?

Answer: C — By a single control device

Braking systems on combination vehicles are commonly controlled by a single control device. This means that the driver applies the brakes through one main control, often a pedal, which engages the braking systems of both the tractor and the attached trailer simultaneously. This design provides a unified and efficient braking response, ensuring that the entire rig slows down cohesively rather than requiring separate actions for each component. Using a single control device is particularly important for maintaining stability and control during braking, as it helps prevent situations where the trailer might react differently than the towing vehicle, which can lead to loss of control or jackknifing. In contrast, options suggesting synchronization would imply a complex interaction not typically manual for the driver, while separate manual controls would not provide the necessary cohesive braking effect. Likewise, relying solely on automatic systems neglects the driver's essential role in managing vehicle operation, especially in emergency situations.