a) What type of study is this?
b) Can this study be used to conclude a causal relationship between increased stress and muscle cramps?
No, because it is an observational study.
c) State possible confounding variables that might explain the observed relationship between increased stress and muscle cramps.
Maybe the students get stressed around exam time.
Since it is exam time, they study more, so they sleep less and drink more coffee to be able to study more.
They also sit more because they study more, so they might get muscle cramps from the inactivity.