Answer:
The correct answer is - option B. he should only change the wavelength of light for each group.
Step-by-step explanation:
In any experiment or research, there should be only one variable that needs to change to see the effect of that variable on the dependent variable and other variables should be constant so there no other variables could influence the result of the experiment.
In this experiment, there is two variable are manipulated that are the different wavelength of light which is desired variable or independent variable in this experiment but CO2 level also changes in each group which should be constant but now it affects the experiment.