Answer:
A. The starting height of the ball
Step-by-step explanation:
When we talk about controlled variables, we refer to the variable that should be kept the same throughout the experiment. The reason why we do this, is to limit anything else that is not being tested, that may affect the results of the experiment.
In the scenario given, the experiment is to see the relationship between the initial height of a basketball and the height of its rebound bounce.
So you the starting height of the ball should vary, meaning it is NOT controlled.