Question

What literary device is used in these lines from "Music, When Soft Voices Die (To--)" by Percy Shelley?

Answer 1

If the options are metaphor, alliteration, allusion, and apostrophe, I believe the answer is metaphor, because the others don't fit.
Alliteration is the repetition of a consonant.
Allusion is reference to something or somebody outside of that literary work.
And apostrophe is the author speaking directly to somebody outside the poem, usually gods or a muse. 

Answer 2

for plato users the answer is alliteration