Final answer:
The theme in Walt Whitman's poem 1 of “Song of Myself” is that all humans share the same origins. The line that best develops this theme is, “For every atom belonging to me as good belongs to you.”
Step-by-step explanation:
In poem 1 of “Song of Myself” by Walt Whitman, the theme that is developed is the idea that despite its diversity, humankind shares the same origins. Whitman emphasizes the interconnectedness of all people, suggesting that each individual is linked to every other through a shared essence. He uses his own experience and being as a metaphor for a universal connection, exemplified in lines like, “For every atom belonging to me as good belongs to you.”
Looking at Part B, the line from the poem that best develops this theme is “For every atom belonging to me as good belongs to you.” This line poetically affirms that every part of the speaker's being is equivalent to that within every other person, thereby reinforcing the theme of a fundamental human kinship.