Answer
At the highest point of the motion, the acceleration is $g$.
This does not depend on whether air resistance is negligible or not.
Work Step by Step
At the highest point of the motion, the acceleration is $g$.
At the highest point, the velocity is zero, so the only force acting on the object is the force of gravity which results in an acceleration of $g$.
Note that this does not depend on whether air resistance is negligible or not. Since the velocity of the object at the highest point is zero, there would be no air resistance even if air resistance was non-negligible.
