Answer:
C) Mass of the ball
Step-by-step explanation:
Independent variable is the variable the researcher changes.
Since Martin is testing the mass of the ball, he'll be using different balls and that is the only thing he changes.
The distance traveled by the ball is the dependent variable since it depends on the mass of the ball.
The height and length of the ramp are the constant variables since that's the only ones that remain the same throughout this experiment.