The dependent variable is the mood of the participants.
The study aims to assess how the level of exercise (measured by the amount of time spent) affects another variable, that, in turn, will be the dependent variable: the mood of the participants. The aim of the study is to conclude whether it is possible or not to infer causality between those two, in other words, to see whether different levels of exercise (the cause), trigger the emergence of different types of mood (the effect).