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.
Oh! the old swimmin'-hole! whare the crick so still and deep
Looked like a baby-river that was laying half asleep,
And the gurgle of the worter round the drift jest below
Sounded like the laugh of something we onc't ust to know
Before we could remember anything but the eyes
Of the angels lookin' out as we left Paradise;
But the merry days of youth is beyond our controle,
And it's hard to part ferever with the old swimmin'-hole.
Intro- within
Extro- outside of
FYI synonyms are words that have the same or almost the same meaning, antonyms are words that are completely opposite.
The answer is C.
Hope it helped.
-Charlie