For Main Point questions, I often ask myself, "what was the most valuable sentence?"
If there is one sentence that feels most like the overarching thesis, or the author's signature opinion, then there is a good chance that the correct answer will paraphrase that line.
Otherwise (on in addition), I ask myself, "what was the purpose of this passage?"
Thinking about what the author was trying to accomplish/convey with the passage forces me to focus on the big picture idea.
For this passage, there isn't one sentence that stands out to me as the most important. Lines 33-41 seem to be the most important excerpt because they are Cullen's response to criticism / articulation of purpose.
Many Arts/Humanities passages discuss an artist's work in light of a common criticism of that artist. (Normally I think of these passages as "Defend Against Criticism" passages, because the author normally tells us about a common criticism and then proceeds to either defend the artist or explain how the artist defends himself/herself).
However, the structure of this passage isn't quite like the normal "defend against criticism" structure, because this passage goes on to give us a neutral description of the artist's later work that has nothing to do with the criticism in the 2nd paragraph.
A) This answer puts the main focus on Cullen's later work. While everything it says is true, it didn't seem like the passage's focus was on Cullen's later work.
B) This answer puts the main focus on a consistent concern Cullen had with racial issues. Indeed, concern with racial issues was a theme in the 2nd and 3rd paragraphs. It acknowledges the European forms (which permeate the 1st and 2nd paragraph) and the religious themes (which permeate the 1st and 3rd paragraph). So overall, it is hitting on all the major points.
C) This is contradicted by Cullen's thoughts beginning around line 33.
D) "primary goal" is too extreme and unsupported. Cullen wanted his racial/political concerns to go hand-in-and with his artful crafting of poetic forms. He never ranked one above the other.
E) This could be tempting, because religious and racial themes were both frequently discussed. However, they were never related to each other the way (E) describes. In fact, the 3rd paragraph begins by suggesting that there was a shift away from racial issues and towards religious ones, not that the religious aspects illuminated the racial ones.
The correct answer is (B). To me, it is somewhat surprising that it doesn't directly address the criticism brought up in the 2nd paragraph, but it is certainly a true answer and it relates to things mentioned in all three paragraphs.