D. To establish that the position stated in the document is legal
The answer is D. Nationality
The best answer here is chronologically. If we read the excerpt, we can see that the author is starting at the beginning and following a timeline to describe each thing about Gertrude Belle Elion. The other options don't make much sense because there are no interviews cited, though there was obviously come research done. Cause and effect doesn't make sense because there isn't any sort of indication of the causes and effects of certain things except at the end when we find out she never earned her doctorate. The same holds true for problem-solution.
