The argument can be made that the hunter is (d) none of these. Instead, the hunter is a stock character because of the fact that he simply is in one episode, and he called a common noun--"a hunter," whereas flat characters usually have names, proper nouns.
In addition,stock characters act and speak in stereotypical ways. The hunter is the white man with the rifle and dog out for one purpose, uncaring about anything else along the way. While he does lift Phoenix Jackson out of the ditch, he tells her to go home because it is too great a distance to town for her. When the old woman demonstrates her determination, he merely laughs and ridicules her: " I know you old colored people. Wouldn't miss going to town to see Santa Claus." He, then, turns and gives his attention to his dogs, who, of course, are more important to him. When he turns around and Phoenix is still there, he points the gun at her (for fun, as he has probably done to other blacks), asking her if it scares her. Phoenix says it does not, so his "fun" is over; he turns and leaves, unconcerned, typically.
I think that the answer here is clearly B -- he is a flat character.
The reason for this is that one definition of "flat character" is a character who has only a minor role in the story and does not undergo any serious change in the story.
He is clearly not the protagonist because he only appears for a little while and is really quite hostile and unpleasant to Phoenix, who is the protagonist.
He cannot really be a round character because he is not around enough to change in any way. We see him being an arrogant racist the whole time that he is present.