Potential energy is the energy possessed by an object due to its height. And kinetic energy is the energy possessed by an object due to the motion of the object.
Thus the potential energy will be maximum when the roller-coaster is at point A and point C as they have the maximum altitudes. As the roller coaster starts to move down the track, it will lose its height and gain speed. Thus, as it moves down, it will lose potential energy and gain kinetic energy.
But the total energy of the roller coaster will remain the same due to the law of conservation of energy. Which states that the total energy of any system will always remain the same.
Therefore,
At point A car would have more potential energy than point B. This is because point A has a larger altitude. As the car moves down the track from point A the potential energy is converted into kinetic energy. The total energy remains the same because of the law of conservation of energy.
