At c: f? mg=mv2R？ ②
According to ① ② formula: F=2mg, that is, the force that the plate needs to exert on the ball at point C is greater than that at point A, so A is wrong.
B, the ball in motion, the kinetic energy is constant, the potential energy is always changing, so the mechanical energy is not conserved, so B is wrong.
C, the speed of the ball is equal at any moment, that is, the minimum speed of the ball at the lowest point C is equal to the minimum speed of the highest point.
According to mg = mv2r and v = gr, so c is correct.
D, providing centripetal force according to the horizontal component of gravity, namely mgtanθ=mv2r.
Therefore, v=grtanθ, so the inclination θ between the plate at B and the horizontal direction increases with the increase of speed, so D is wrong.
So choose: C.