The Cartwheels cause friction.
Option A.
Explanation :
The cart on the roller coaster works on the principle of conservation of energy which says that the energy is only transformed from one form to another. It is not lost.
When the cart with wheels on the roller coaster goes up, the gravitational potential energy increases with height and is the highest at the highest point on the roller coaster. As the cart comes down, the potential energy is converted into kinetic energy. The kinetic energy is the highest at the bottom of the coaster.
Friction is offered by air and the wheels. That is why the roller coaster will not go on forever unless it provided some external energy.