According to Quantum Mechanics, the ball can be considered to be at the top of its path as well as everywhere else at the same time. There is only a higher probability of finding the ball at the top of its path at any given moment. You need to calculate the Wave Function of the Ball to work out the probability of the force acting up and down at the same time.
all u need to know is that air resistance affecting an object increases as the velocity of the object increases. so when the object in the question is at its maximum height, no air resistance affects it. so the only forcing acting on it at that moment is f= mg so regardless if u factor in air resistance or not, the answer is f = mg