Answer:
D
Step-by-step explanation:
Comment
C is the second best answer, but it is not compared to anything, so you don't know if the tree is acting well in the environment, or not so good at all.
D is the best answer. Take a bunch of trees. Treat them the same way and see what the range is. If it's narrow, then the scientist has not learned that much except that the trees tend to act the same way under the same conditions. If there is a lot of variation, then he or she has to devise another experiment to find out why.