Final answer:
When going down a steep hill in an automatic car, it is typically best to use Drive (D) to allow for engine braking and control the vehicle's speed safely.
Step-by-step explanation:
When descending a steep hill in a vehicle with an automatic transmission, the correct gear to use would usually be Drive (D). This allows the engine and transmission to work together to provide engine braking, which helps control the speed of the car. Using first gear or reverse could potentially damage the transmission or cause you to lose control, while neutral would take away the engine's braking ability, relying solely on the brakes to decelerate, which can lead to brake failure due to overheating. It's important to consult your vehicle's owner manual since some vehicles have special gears or modes for such conditions.