Answer: Size of each leaf.
Step-by-step explanation:
The independent variable is the variable that is changed or controlled during an experiment in order to see what effect it has. The variable that responds to the change in the independent variable is the dependent variable. So the dependent variable is what is being measured or tested in an experiment.
In the experiment, the botanist is studying the effect of different lengths of light to see the size of the leaf which is being measured.
So, the independent variable is the lengths of artificial daylight and the dependent variable is the size of each leaf.