A, the small ring is acted by gravity and the elastic force of the metal rod at point D. The resultant force is not zero and the acceleration is not zero, so A is wrong.

B, small ring at point B and D have different forces, different resultant forces and different accelerations, so B is wrong.

C, mechanical energy is conserved in the process of small ring motion, and the kinetic energy at point C is the largest, so C is correct.

Conservation of mechanical energy in the process of small ring motion. Because the small ring has a certain initial velocity and the kinetic energy of point E is not zero, D is wrong.

So choose C.