I'm going to take a crack at explaining why it is A over D. Hopefully someone from PS can correct me if I'm wrong.
Going into this question, the central point the point the author, through the use of a critic, points out is that using the techniques historians currently use in classifying African art to specific tribes based on the artwork's style, is problematic.
A is correct as it not only captures the aforementioned idea, but it also brings in the theme of African Centers of style which is what the author primarily uses to articulate their ideas.
D is incorrect because of the phrase "Style versus TECHNIQUE". Unless I completely missed it, the author while the author does write about style, they do not mention technique in the passage nor is style compared to technique(s).
Hope this helped