At the highest point of the ball's trajectory, its vertical velocity becomes zero. Since the ball is thrown vertically upward, the acceleration at the highest point is equal to the acceleration due to gravity acting in the downward direction.
The acceleration due to gravity near the Earth's surface is typically approximated as 9.8 m/s². Therefore, at the highest point of the ball's trajectory, the acceleration would be approximately 9.8 m/s² in the downward direction.