The correct answer is the control group. Whenever you have an experiment, you must have two different groups. Every condition between these groups must remain the same, except for the one thing you're testing (also known as the variable). For example, if an experiment concerned athletes' performance with protein drinks, you would need two groups of athletes. Each group would need the same number of athletes, who would be given the same amount of practice time and recovery, as well as the same diets. Since protein drinks are the variable, the control group would not have protein drinks, while the experimental group would. If there are any differences between the two groups (for example if one group performs better than the other), it can be deduced that the difference is because of the protein drinks, since all the other aspects of the experiment were the same between the two groups.
