Answer:
The type of light in the rooms.
Explanation:
In order to find the relation between two variables it has to be observed that one variable changes because of the change in the other variable.
The variable which changes with respect to the other variable is called the dependent variable.
The variable which changes irrespective of the other variable is called the independent variable.
Here, it is being studied what type of light in the room affects the mood of the participants. Hence, the type of light in the rooms whether light or dark is the independent variable while the mood is the dependent variable.