Answer: D. The angle at which the mirror reflects the light from the ray box.
Step-by-step explanation:
In a particular experimental setup, an independent variable is the variable which can be manipulated manually the affect of this is observed on the dependent variable. A dependent variable cannot be changed manually instead it changes due to affect of changes on the independent variable. Generally, the dependent variable is the outcome of the experiment.
According to the given situation, the angle at which light stricks the ray box and then finally stricks the mirror is the independent variable. Now the light reflected by the mirror after this process at a particular angle is the dependent variable, that is the outcome of the experiment.