Final answer:
The most clearly a secondary source is option D) An academic research paper analyzing historical data.
Step-by-step explanation:
The most clearly a secondary source is option D) An academic research paper analyzing historical data.
A secondary source is a document, object, or other source material written or created after the time period under study.
In this case, an academic research paper is a secondary source because it is created after the events have occurred and provides an analysis of historical data gathered from primary sources.