I also narrowed it down to C and E, so I'll explain how I chose E. My main reason was that E includes the methods of research. The author spends the last three paragraphs explaining how he/she approached this project. When asked about the author's primary concern, we do not want to leave out a majority of the passage.
I don't think there is necessarily anything wrong with "summarizing the main points." However, I think that "explaining the author's choice of subject matter" is a better description of the first paragraph. The author is saying that there has not been enough research into the life of Lorenzo Tucker, and yet he is a very important figure in film and theater. The author is explaining the motivation for taking on this project.
