A, in the process of the ball falling, the tension of the rope does negative work, and its mechanical energy decreases, so A is wrong.

B, because the block has to overcome friction to do work, the mechanical energy of the system composed of the block and the ball is not conserved, so B is wrong.

C. If the ball drops at a uniform speed and the kinetic energy of the system remains unchanged, according to the conservation of energy, the reduced gravitational potential energy of the ball is equal to the heat generated by the friction between the block M and the desktop, so C is correct.

D. If the ball drops at an acceleration, according to the conservation of energy, the mechanical energy reduced by the ball is equal to the sum of the heat generated by the friction between the block M and the desktop and the kinetic energy increased by the ball and the block M, so the mechanical energy reduced by the ball is greater than the heat generated by the friction between the block M and the desktop, so D is correct.

So: CD.