Watch television! It's way more entertaining and will not cause stomachaches, tooth decay and cavities lol
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.