The correct answer is A. That his body has dissolved into dirt.
Whitman is writing about the transience of life, and how everyone will have to die at one point. When he says that he is just the dirt under somebody's soles, he means that he will turn to soil once he is dead, and that he will be one with nature, just like everyone else. B and C are silly choices, and D is wrong because he is also praising himself in the poem rather than undermining his own worth.