@joshuavt, the passage is talking about the way canaries learn specific songs from their elders through imitation but then seem to unlearn and relearn those songs and compares that to human babies learning to talk through mimicking adults. So that's the basic framework in which the question is asking about the purpose of the word "vocabulary." When faced with a question like this, you'll likely need to proceed through process of elimination.
(A) nothing suggests canaries have grammatical structure but simply mimic other canary's songs, so this doesn't work.
(B) there's no suggestion that canaries are learning words in the sense that humans do but rather that they're mimicking sounds. So this doesn't work either.
(C) the passage actually contests this, so this doesn't work.
(D) this most accurately describes the say "vocabulary" is used. For humans, our vocabulary is the collection of words that we know and use. Using vocabulary to describe canary songs suggests that they have a repertoire of songs that they know and use much the way humans know and use words.
(E) there is no suggestion that canaries command a vocabulary nearly as large as basic humans, but there's also nothing to suggest that their vocabulary isn't larger, either. So this doesn't work.
