After the ball leaves the woman's hand, the only relevant force acting on the ball is the gravity, which produces an acceleration of a=g=9.81 m/s^2 on the baseball, directed downward.
Gravity acts on the ball for the whole duration of the motion, so the acceleration of the ball is [tex]g=9.81 m/s^2[/tex] at every point of the motion, and therefore also when the baseball is at its highest point.
