I would interpret it as how people only look at reality, not what's inside or what they can't see.
The answer is A. There is a metaphor and a simile. I hope that this helps. =)
In this poem, Pound is comparing "faces in the crowd" to petals that are laying against a tree branch.
This is exactly what he says in the poem - "petals on a wet, black bough." A bough is the main branch of a tree, so this is definitely the only appropriate answer in this case. He wants to say that people resemble these petals.
