If the vertical acceleration of an object at its maximum height is zero, it means that the object has reached the peak of its motion and is momentarily at rest before falling back down. At this point, the velocity of the object is zero.
In projectile motion, the vertical velocity of an object changes due to the influence of gravity, which causes it to accelerate downward. As the object rises, its velocity decreases until it reaches zero at the maximum height. After that, it starts descending, and the velocity increases in the downward direction.
So, at the maximum height, the velocity of the object is zero.