I agree, it's kind of a dumb answer, since the "extra" parallels between the structure of the novel and Ellington's music still ultimately relate to the first person / third person shifting.
Essentially, they're referring to the fact that this 1st/3rd person shifting is well balanced in relation to the whole.
Lines 24-27 are saying that throughout these shifts, the narrator "is both generous with the characters' voices and protective of his or her masters over the narrative as a whole".
Lines 39-42 are saying that throughout these shifts, "no matter how lengthy his musicians' improvisations ... they are always performed within the undeniable logic of the composer's frame".
So it's not JUST that the novel has some 3rd person sections and some 1st person sections, it's ALSO that the 1st person sections fit within the frame of the 3rd person sections.
This is reinforced once again in 49-52, "the sense of an ensemble of characters improvising within the fixed scope of a carefully constructed collective narration".
Again, kinda dumb.
I think it's easiest to get this through elimination, although finding these line references is what allows me to make peace with my answer.
== other answers ===
(A) no way to justify "little success by other NA writers in 20th century"
(B) no way to justify "the MOST successful representation"
(C) no way to justify "difficult to distinguish between voices"
(E) no way to justify "disguising the connections with Ellington"
Hope this helps.