To accurately determine the best answer, it would be helpful to know more about the content of the article. However, based on the answer choices provided, the overall purpose of an article discussing the decline of big trees might aim to either bring awareness to the issue, explain contributing factors, or foster enthusiasm for their protection.
If the article primarily focuses on raising awareness about the decline and its mysteries, option A may be the best choice. If it explains factors contributing to the decline, option D may fit better. If it focuses on conservation enthusiasm, then option C could be correct.
If you can provide more context or key points from the article, I may be able to help pinpoint the best answer more accurately!