Answer: A) It emphasizes the narrator’s belief that every parcel of the Earth belongs to everyone.
Step-by-step explanation: An atom is the smallest particle of a chemical element that can exist, so in the given line from "Song of Myself" by Walt Whitman, the narrator says that every atom that belongs to him, belongs to the other person too, he uses the word "atom," that is the smallest particle that exists, to emphasize his belief that every parcel of the Earth belongs to everyone, so the correct answer is option A.