An experiment was conducted to determine how thick a concrete wall needs to be to block out the sound of highway traffic. which of the following variables must be controlled (held constant) in the experiment to get the best results

Only one sound blocking material should tested at once and the sound will vary on how much and what kind of materail you use.

The material will need to be held constant because the experiment was to determine the necessary thickness of a CONCRETE wall. The height will also be a controlled variable, as will the sound. In short, the only variable that will change is the thickness of the wall.