A: During the whole descending process, the magnetic flux passing through the copper ring has been changing, so the induced current is generated and some electric heating is also generated. The total energy is equal to the mechanical energy of electric heating and copper ring, and the total energy is conserved, so the mechanical energy must not be conserved. So, A is wrong.

B: ① When the copper metal ring falls to the S pole, the magnetic flux passing through the copper ring first increases and then decreases; ② From the S pole to the midpoint of the magnet, the magnetic flux passing through the copper ring increases; ③ From the midpoint to the ground, the magnetic flux passing through the copper ring decreases. According to Lenz's law, the force of the magnet on the copper ring is up and down, that is, the force of the magnet on the desktop is not always greater than that of the magnet.

C: During the whole descent, the magnetic flux through the copper ring has been changing, so there has been induced current and electric heating. Moreover, the total energy is equal to the mechanical energy of electric heating and copper ring, and the total energy is conserved, so the mechanical energy of copper ring has been decreasing. So, c is wrong.

D: During the whole falling process, the gravitational potential energy decreases, while the kinetic energy and electrothermal energy (internal energy) increase. According to the conservation of energy, the decreased gravitational potential energy is equal to the increased kinetic energy and electrothermal energy (internal energy), so the increase of kinetic energy is less than the decrease of gravitational potential energy during falling. Therefore, d is correct.

So choose: D.