The question is incomplete and the full version can be found online.
Answer: The change in IQ score from the first to second test
Step-by-step explanation:
In an experiment, the response variable is the purpose of the question that is being made, while the explanatory variable is the one that has an effect over the response variable, meaning that it explains any changes in it.
If Brian is examining the effect that exercise might have on a person's intelligence quotient (IQ), the question he´s making is: does exercising change IQ results? Therefore, the change in IQ is the response variable. The amount of exercise is the explanatory variable, and the experiment aims to determine whether or not it has an effect on the response variable.
Incorrect options: The number of people in the study is the sample under study, not the response variable. The first IQ test score would be one indicator of the variable, not the variable itself. And the amount of exercise the people do is the explanatory variable, not the response variable.