The most likely reason the author chose a free verse structure for "Song of Myself" is that:
Free verse allowed for the expression of expansive ideas.
Walt Whitman in his poem “Song of Myself” prohibits himself from singing the songs of war, rage, and journeys and instead sings the song of himself as the subject of his poem. The poem deals about his journey and experience in this world. There are two main characters in his poem, which are 'I' and 'you.' Whitman speaks about the grass which is the symbol of democracy. He says that the grass of democracy feeds itself on the bodies of the dead. In the poem Whitman used stanzas of varying length and the topic of his poem changed with change in stanzas.
Pretty sure it’s the last two choices
