Final answer:
The combination valve is used to hold off the application of the front brakes on vehicles with disc brakes on the front wheels and drum brakes on the rear wheels.
Step-by-step explanation:
The correct answer is d. Combination valve.
The combination valve is used to hold off the application of the front brakes on vehicles with disc brakes on the front wheels and drum brakes on the rear wheels. It is a component that combines multiple valves, such as the load-sensitive proportioning valve, metering valve, and pressure differential valve, into one unit.
By using the combination valve, the braking force is distributed between the front and rear brakes to ensure proper braking performance and prevent premature rear wheel lock-up.