Primary purpose, main point, and "title" questions all are asking the same thing: why did the author write this passage? In an overwhelming majority of passages (90%+), the author presents an idea, and then gives their opinion on that idea. If the author is silent/missing/invisible, the primary purpose is just a summary of the passage.
Here is my strategy every time, I look for the first layer, "the idea" then I wait for the author to give their opinion on that idea. We see the idea right at the top, almost always, and here it is the same: there are three basic ways organisms adapt. Now that I have that idea, I am reading the rest of the passage, anticipating that eventually the author will say, "here is what I think of that..." But in this case, it never happens. So, I just have the main idea only: "there are three types of ways an organism adapts." That already matches (D), while (D) goes deeper than my anticipation "...and provide an example..."
What is wrong with (A)? It does not discuss the main idea at all, and skips to the end, the illustration. Further, (A) is technically incorrect as well, there is no indication that environmental conditions are changing and leading to responses -- winter comes and goes every year, and bugs are born with and without these types of wings every year. It is not an environmental change.
Hope that helps and happy studying,
Nate, LSATMax Instructor