A scientist conducts an experiment to test the effects of light on earthworm activity. Which is a variable in this experiment?

They are all variables. In an experiment you have independent and dependent variables. The dependent variable changes as a result of a change in the independent variables. So in your example the earthworms activity would be the dependent variable and the light intensity, soil conditions, and temperature would be the independent variables. You could manipulate any of the 3 independent variables and it may change the level of activity of the earthworms (the dependent variable). Very important in an experiment that you only change 1 independent variable at a time so that you know which change is having an effect on the behavior!

The light is the variable.