Answer:
Members of his family were killed by Nazis during World War II.
Explanation:
The significance of Anne Frank's diary is because of the historical background of the Holocaust, the most famous genocide in the world's history. And though written by a young girl, the book was also a first-hand account of the whole debacle.
Eli's strong opinions about the Diary and his desire that his class takes the books seriously would stem from the fact that members of his family were killed by the Nazis. This would make him relate to Anne Frank's accounts in a personal way.
Thus, the correct answer is the third option.
