Answer:
d. natural selection
Step-by-step explanation:
Natural selection is the main process of evolutionary change which involves the selection of individuals with favorable traits that increase their survival and reproduction against individuals that do not possess these favorable traits. Hence, the individuals that have these favorable traits become more adapted to their environment, and thereby produce more offspring with these traits from generation to generation. Over time, individuals with traits that make them less adapted and fit to survive in the environment soon become fewer and disappear. Only individuals with favorable traits become better adapted and more common in the environment.
Therefore, we can conclude that natural selection is the most likely process that brought about the change in the plant population stated in the question.