A, the object first makes uniform deceleration motion and then makes uniform acceleration motion. The displacement-time relationship diagram is a parabola, not a straight line, so AB is wrong;

C, the object first makes uniform deceleration motion along the inclined plane, and then makes uniform acceleration motion along the inclined plane. According to Newton's second law, the accelerations of the two processes are the same, both of which are downward along the inclined plane, so the motion of an object can be regarded as a straight-line motion with uniform deceleration along the inclined plane. Because the acceleration is constant, the slope of v-t image is constant, and the speed is positive first and then negative, so C is right and D is wrong.

So choose: C.