When an object is at any height above the ground, it has gravitational potential energy. If the object falls, it loses this gravitational potential energy and this energy is converted to kinetic energy. By this logic, we can say:
Gravitational potential energy of grain sack = Kinetic energy of grain sack
As the ball falls from C to E, potential energy is converted to kinetic energy. The velocity of the ball increases as it falls, which means that the ball attains its greatest velocity, and thus its greatest kinetic energy
