Answer:
The age of the participants
Explanation:
Confound is described as one of the different methodologies that are responsible for creating confounding variables. Although, it can often threat the results of particular research because of its relationship with that of the independent variable as well as the dependent variable.
A confound is a variable that an experimenter intentionally doesn't control i.e, the extraneous variable and often influence the findings of the research.
In the question above, A major confound in Madison's experiment is the age of the participants.