When you're driving a. it is much easier to stop your car as opposed to driving downhill b. it is more difficult to stop your car as opposed to driving downhill c. you need to double your speed?

The answer would be:

A. It is much easier to stop your car as opposed to driving downhill.

Let me explain why:

When you are going uphill, it is easier to stop your car because gravity is helping you out. Gravity is a downward force, which means that gravity is pulling against your upward movement, which slows down your car. It is difficult going down because gravity is still pulling you down along with your forward movement.