While the stone is ascending, its speed decreases due to gravitational attraction and air resistance. Although gravitational attraction is constant, air resistance decreases with speed. Therefore, the total force opposing the motion decreases. This results in a decrease in deceleration.
When the acceleration decreases, the change in velocity gets lower for a constant change in time. Thus the gradient should decrease. Additionally, the velocity of the stone must be zero when it reaches maximum height. These two conditions are satisfied in choice C.
