Answer:
The study guide.
Explanation:
It's the dependent variable because is the one she's manipulating in the experiment. With the other variable 'perform better on exams' she only measures it.
Remember that the relation between variables is that you manipulate one (dependent) to study its effects in the other one (independent).
You cannot manipulate independent variables to study its effects on dependent ones, because it doesn't make sense, since that independent variables can't be manipulated.