A variable refers to anything, which is being modified within an experiment. A dependent variable refers to the variable whose value relies on the independent variable. The independent variable refers to the variable, which is being manipulated by the experimenter.
In the given experiment, the student is experimenting that how the height from which the ball is dropped influences the bounce of the ball. The height at which the ball bounces is the variable that is being determined and relies upon the height from which the ball is dropped, that is, the independent variable.
Hence, the control variable would be the ball used.
