When the girl lifts the ball from waist level to above her head, she is increasing the ball's potential energy. This is because potential energy is associated with an object's position in a gravitational field, and lifting the ball raises its height, which increases its gravitational potential energy.
So, the correct choice is:
B. increasing the ball's stored energy (potential energy).
(It's worth noting that "stored energy" can refer to potential energy in this context.)