The sentence referred to in the question makes the following claim: "such binary oppositions are used analogously in West African tales, and it seems likely that these dialectical elements are related to African oral storytelling more than, as many critics have supposed, to the Marxist components of his ideology." Which of the following facts would strengthen this claim?
Let's look at (A) and (E)
(A) Several African novelists who draw upon the oral traditions of West Africa use binary oppositions as fundamental structures in their narratives, even though they have not read the Marxist theory.
(A) tells that binary oppositions play "fundamental" role in the narratives of novelists who draw upon the oral traditions and are completely unfamiliar with Marxist ideology, suggesting that it is more likely for one to use binary oppositions because he is inspired by the oral tradition rather than Marxist ideology.
(E) Binary oppositions do not play an essential structuring role in the narratives of some films produced by other filmmakers who subscribe to Marxist principles
The issue with (E) is that even if binary oppositions do not play an essential role, they could still play a role thus weakening the argument. Further, it only tells us that binary oppositions do not play an essential role in "some films," leaving open the possibility that they do play "an essential role' in most films, further weakening the argument.
Let me know if this makes sense and if you have any further questions.