Which system includes a hydraulic control unit?

The correct answer is the antilock brake system (ABS) because this system relies on a hydraulic control unit to manage the braking process. The hydraulic control unit is crucial for modulating the brake pressure during an ABS event. This allows the system to prevent wheel lockup during hard braking by rapidly applying and releasing brake pressure, thereby maintaining vehicle control and traction. The presence of the hydraulic control unit differentiates ABS from other systems that may use mechanical linkages or different forms of control.

In contrast, while the transmission system, steering system, and suspension system may contain hydraulic components, they do not primarily function on a dedicated hydraulic control unit like the ABS does. The transmission system mainly deals with gear changes, the steering system focuses on directional control, and the suspension system is concerned with ride comfort and vehicle stability. Therefore, the specific function of a hydraulic control unit in modulating brake force in the ABS is what sets it apart and makes it the correct answer.