The cup moves in a uniform circular motion on the vertical plane. Although the speed is constant, the direction of motion is constantly changing, so the state of motion is constantly changing. So, d is wrong.

Because the motion state of an object will not change under the action of a balanced force, the force on the cup is unbalanced, so A is wrong.

The cup moves in a uniform circular motion in the vertical plane, and the height of the cup changes constantly, so the gravitational potential energy of the cup changes constantly. But the speed is constant and the mass is constant, so the kinetic energy is constant, so the mechanical energy is constantly changing. So, B is wrong and C is right.

So choose C.