The author includes a few references to the characters of Sembene's films. Consider the examples on line 23: a thief, a corrupted civil servant, and a member of the elite. Also, consider the protagonist of Borom Sarret, who is a street merchant (line 31). These examples, in addition to the discussion of social injustice in the last paragraph, give me enough information to select D. The passage didn't use the phrase "social strata" specifically, but the difference between the street merchant and the member of the elite provides the broad range we are looking for.
