The correct answer is "reciprocity."
Reciprocity signifies a mutual exchange in which both parties benefit. In this case, Maddie is one party, and her professor is the other party. Since Maddie is volunteering to work with participants in her professor's study, her professor is benefiting from this free work. To make this a reciprocal relationship, Maddie would then have to receive a benefit, such as extra credit on her grade.