A, the golf ball moves in a curve from point B to point D, and the motion state changes constantly, which shows that the golf ball is affected by unbalanced force, so A is wrong.

B. When the golf ball rolls on the grass, it is subjected to friction and other resistance, so the mechanical energy of the ball is decreasing (the reduced mechanical energy is converted into internal energy). The mechanical energy of point D is less than that of point B, and the heights of point B and point D are the same, so the gravitational potential energy is equal, so the kinetic energy of point D is less than that of point B, and point B is wrong.

C. When the golf ball rolls on the grass, it is subject to resistance such as friction, so the mechanical energy of the ball is continuously reduced (the reduced mechanical energy is converted into internal energy), so the mechanical energy is reduced in the process from B to C, and C is correct;

D, the magnitude of gravitational potential energy depends on the quality of the problem and the height of the bump. The quality of the golf ball remains the same, and the height from point C to point D increases, so the gravitational potential energy increases, so D is wrong.

So choose C.