The writer reveals information about a character through his personality, thoughts, words, and actions. Also, writers include of how they feel and how they speak and act towards the other characters.
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 "All of the choices are correct"
Your answer is A. I hope I help and I hope you get a good grade!!