Final answer:
The control variable for this experiment would be the length of the string.
Step-by-step explanation:
The appropriate control variable for this experiment would be The Length of the String. The control variable is the factor that is kept constant throughout the experiment to ensure that any changes in the dependent variable (in this case, the time it takes for the ball to make one complete period) are solely due to the independent variable (in this case, the length of the string).