The independent variable in this case would be the wavelength of the light shining on a plant.
Explanation:
Independent variable means means a variable, whose variation does not depend on that of the other.
The light shining on the plant is independent variable, as its not affected by the rate of photosynthesis or the growth of the plant.
The rate of photosynthesis is dependent on the light shining on the plant, according to the hypothesis and the growth of the plant is also dependent on the intensity of the light shining on it.
The light shining on the plant is independent of what its wavelength or its intensity is.
