the answer should be: The study's construct validity
A study's construct validity refers to the degree to which the experiments made in the study actually measures what it intended to measure. This can be seen on the study's standard of measurement and the correlation between variables used in the measurement and the actual phenomenon.
After hearing the claimed made by the teacher, Clarissa's immediately question whether the act of bullying can be measured.
She brought up an important point. The teacher never really specify what act considered as 'bullying'. Depending on people's perception, some consider a slight discomfort from social interaction might be considered as bullying, while other people might have the threshold. Because of this, she can say that the study's construct validity is questionable.