Answer
(a) a = -g
(b) a = -g
(c) a = -g
Work Step by Step
$g = 9.80~m/s^2$
Let up be the positive direction. Then down is the negative direction.
(a) A person throws a ball up in the air and the ball has left the person's hand. As the ball is moving up in the air, the acceleration vector points toward the ground because of gravity. So the acceleration is negative. Therefore, $a = -g$
(b) At maximum height, the acceleration vector points toward the ground because of gravity. So the acceleration is negative. Therefore, $a = -g$
(c) As the ball falls toward the ground, the acceleration vector points toward the ground because of gravity. So the acceleration is negative. Therefore, $a = -g$
