Question: What is the acceleration of the ball at its maximum height? [When a ball is thrown vertically upward]
Answer: Acceleration working on the ball at this point of maximum height is the acceleration due to gravity (g).
At this highest point of the vertical motion, the ball is ready to fall vertically downwards with acceleration g. g is used with a positive sign in motion equations during this free fall (ignoring air resistance). As the downward motion of the ball is along the same direction of gravity, hence the motion will be one with acceleration.
Also please note that: It is the point of maximum height where the velocity of the ball becomes zero.