Answer:
B). To compare the speaker to a force of nature.
Step-by-step explanation:
Simile is described as the literary device or figure of speech that is employed to compare two things with an aim to reflect similarity between them with the use of 'like' and 'as'.
Whitman's "Song of Myself" is illustrated as one of his most acknowledged works. The poem is chiefly inclined towards celebration of individualism(as the title suggests) and its capability to unite mystically and spiritually with the nature and individual self. Whitman employs different literary devices to create the intended effect efficiently and serve his purpose.
In the given excerpt, the purpose of the simile('I depart as air') is to 'establish the comparison between the speaker and force of nature' that highlights the 'transience' or 'brevity' of human life and ultimate union with the nature. Thus, option B is the correct answer.