Answer:
C) Roll the same ball down each inclined plane, with no additional force, and measure how far the ball goes for each plane.
Step-by-step explanation:
The inclined plane should be the only variable that changes so by using the same ball and amount of force, you are ensuring that these are controlled variables. The question says that you are looking for which inclined plane makes a ball travel the farthest so this makes the last part of the answer correct.