Thanks for the question! So we’re being asked for the author’s attitude with respect to the Cold War here. Now, admittedly, if this were a short answer type of question, you probably wouldn’t come up with (C). But remember, our answer choices should at least have some basis in the text. Where do you see that the author feels “regret” or when it comes to the Cold War, or that the European countries were ambivalent, let alone regret about an ambivalence? (D) is simply not supported by the text and so can’t be the right answer.
Hope this helps! Feel free to ask any other questions that you might have.