Answer:
EXERCISE
Step-by-step explanation:
In an experiment, the independent variable is the variable that the experimenter changes or manipulates in order to bring about a measurable outcome.
In this case, an experiment is carried out to test the hypothesis that "exercise will affect the number of times a person can squeeze a clothespin in a certain amount of time". The experiment contained two groups (one group sits quietly before squeezing a clothespin as many times as possible and the other does 25 JUMPING JACKS before squeezing a clothespin as many times as possible).
The independent variable is the EXERCISE (jumping jacks) that was changed for each group in the experiment.