The correct option is A 4s
Let a be the acceleration of the ball, u be the initial velocity, v be the final velocity and t be the time taken.
Initially the ball was moving in the upward direction. At maximum height, velocity will be zero, (v = 0). At that point, the ball will change the direction of motion and fall downwards.
Since, acceleration due to gravity is opposite to motion, a=−10ms−2
From first equation of motion, v=u+a
⇒0=20+(−10)t
⇒t=2s
Thus, the ball takes 2s to reach the top. Similarly to reach the ground from the top point is also 2s. Thus total time or time of flight is 4s.