B is correct "A social commentator, who explains what a book says about people in general."
I think it would be simple since it only has one subject (it) and one predicate (is).
Including Keffer's memory helps achieve the author's purpose because the author's purpose is to inform, and this informs the reader about pedestrians, and what they should do! It's important to know what Keffer remembered because it helps achieve the authors' purpose, and adds on to more information mentioned earlier in the text. It also gives us Keffer's point of view!
