First, take a look at my passage map:
passage-discussion-t5539.html Let's roll through the answers:
(A) is tempting, as it has Porter showing a relationship between African and African-American art. However, (A) incorrectly represents the passage as being focused on why Porter's work focused on that connection. Instead, the passage is about his work, not the reasons for his work.
(B) is also quite tempting. However, this answer is unsupported - we never learn what is Porter's most important achievement. Furthermore, we never learn about African-American art connecting to other art in America; the whole passage is about connecting African Art and African-American art
(C) is contradicted by lines 26-32.
(D) is contradicted and/or unsupported - while we learn that Porter left some unfinished writings about African-American art and its relationship to Western culture, there's no indication that this is the main focus of his work.
(E) is correct. It's supported in various places in the text: 1-6, 26-32.