Final answer:
When driving downhill on a very steep grade, it is safest to use lower gears to help control the vehicle's speed through engine braking, preventing brake overheating and maintaining control.
Step-by-step explanation:
If you are driving downhill and the downgrade is very steep, the most effective way to control the speed of your vehicle is to use lower gears. This technique, known as engine braking, allows the engine to help slow the car, reducing the reliance on the brakes alone which can overheat and become less effective if used continuously. Using lower gears puts the engine in a better position to apply engine braking, which is especially useful on steep descents. Therefore, the correct answer is a. Use lower gears.
Using neutral or increasing speed are not safe practices on a steep downgrade as they prevent you from maintaining control over your vehicle's speed. Applying brakes continuously can lead to them overheating, which may cause brake failure.