When conducting the experiment, the student should keep the height she drops the balls from the same.
When conducting an experiment to determine the effect of mass on kinetic energy, the student should keep the height she drops the balls from the same.
This is because the potential energy of an object is directly proportional to its height above the ground.
By keeping the height constant, the only variable that changes is the mass of the balls, allowing the student to observe the effect of mass on kinetic energy.