At point A, the cart is just sitting there at the top of the highest hill. It is the highest point and is not moving yet, so the gravitational potential energy is at its highest and the kinetic energy is at its lowest. As the cart starts rolling towards point B, the gravitational potential energy is changing into kinetic energy because the height is getting less. That is why the cart starts moving.

As the cart gets to point C, the gravitational potential energy is gone because the height is zero on the valley floor. Therefore, the kinetic energy is at its highest point. As the cart starts moving towards point D, the kinetic energy is changing back into gravitational potential energy because the height is getting greater again. As the second hill is shorter than the first hill, the gravitational potential energy at point D is less than the gravitational potential energy at point A. However, the kinetic energy is still present because the second hill is still greater than the valley floor.