Answer:
The correct answer is option D.
Step-by-step explanation:
In the given experiment the variables are shade and direct sunlight for the experimental group of plants while the shade is variable for all the group of plants that grow best in shade.
According to the given information option, A and option B are incorrect in terms of different variables. Different vegetation requires a different dose of direct sunlight can not be generalized, so option c would not be answer.
Thus, the correct answer is option D.