The dependent variable in this experiment is the temperature of the air in each box.
In an experiment, the dependent variable is the variable that is being measured or observed in response to the independent variable, which is the variable that is being manipulated or changed. In this case, the independent variable is the material used to make the boxes (glass or clear plastic), and the dependent variable is the temperature of the air in each box.
The other variables listed (2 boxes of the same size and type, the sunlight for 1 hour, and glass and clear plastic) are not the dependent variables in this experiment. They are either controlled variables (which are held constant in order to eliminate their influence on the results) or extraneous variables (which may affect the results but are not being studied in this experiment).