The answer to your question is OPTION D
It made more sense in the aspect that,yes Nick was one of the main characters but he was there to see a story unfold. To see who,how and why Gatsby,Daisy or Tom were they way that they were.
The answer would be A
A round character has depth, experiences feelings, emotions, etc. about which the reader is aware. Therefore, a round character is likely to experience internal conflict.