The correct answer to this open question is the following.
Although the question does not provide any options, we can say that Margaret is investigating if gender has any effect on political party associations. The response variable is precisely political party associations.
Why it is the response variable? Because it is the main point of the issue. We can say that in the process of research, "political party associations," should be the focus of the investigation, the main question to study and develop the research form there. In simpler terms, the response variable is the one that asks the specific question in the statement.