Which of the following is a type of auxiliary braking system?

An auxiliary braking system is designed to supplement the primary braking system, enhancing the vehicle's overall braking capability, especially in situations like descending steep grades, where additional control is needed. Among the options provided, the driveline retarder is specifically recognized as an auxiliary braking system. It works by using the vehicle's drivetrain to create resistance, effectively slowing down the vehicle without excessively wearing the primary brakes.

This system is particularly beneficial in maintaining control and reducing brake fade, which can occur from overheating of the primary braking components due to prolonged use. While the engine brake and exhaust brake are also types of auxiliary braking systems, the driveline retarder distinctly utilizes the vehicle's mechanical components for effective braking, making it a clear representative among the given choices.

The hydraulic brake, on the other hand, is part of the primary braking system and does not serve as an auxiliary option; it relies on hydraulic force to activate brake pads against rotors to slow down or stop the vehicle.