The most likely source of error in this investigation is:
She dropped the balls from different heights.
This response indicates that if the balls were dropped from different heights, it would affect the time it takes for each ball to hit the ground, thereby introducing variability in the measurements that is not due to the balls' mass or any other factor. The other options do not directly impact the time it takes for the balls to hit the ground in the context of gravity's effect on different masses.