The most likely source of error in the student's experiment would be that she dropped balls of different sizes. When investigating the effects of gravity, it's important to ensure that the conditions of the experiment remain consistent. Different sizes may lead to differences in air resistance, which can affect the time it takes for each ball to hit the ground.
Dropping balls from different heights could also lead to variations in results, but if the heights are not significantly different, the effect might be less impactful than the size difference. Not measuring the mass or circumference of the balls is irrelevant to the timing aspect, as gravity affects all masses equally in a vacuum. Thus, the primary concern in this scenario is the variation in size, making it the most significant source of error.