In an experiment, the independent variable is the factor that is changed or controlled by the experimenter to test its effects on the dependent variable. In this case, the independent variable would typically be the factor that is manipulated to see how it influences the outcome (which could be any of the other variables).
From the provided options, "the amount of light each plant receives" is the independent variable because it is the factor that can be controlled and changed in the experiment to observe its effects on the tomato plants (like the number of fruit per plant, average circumference of the fruit, or height of the plants).
So the correct answer is:
the amount of light each plant receives.