Thanks for the question! So you’re certainly right that (D) is supported by the passage, so it’s certainly true. But remember, this question is asking us what the author is primarily concerned with. So not only does it have to be true, it has to be a major point of the passage. And (D) just isn’t important enough to be the primary purpose, it’s not the point. (A) is a much better description of what the primary purpose of the passage is, and so is correct.
Hope this helps! Feel free to ask any other questions that you might have.
