In “caged bird,” the poet uses rhyme to emphasize

the caged bird’s song
the free bird’s flight
unpleasant noises
images of nature

The caged bird's song.

Explanation:

Maya Angelou's poem "Caged Bird" tells the metaphorical position of the black people as birds kept in a cage, devoid of any freedom. The poem became a huge success, with its referencing to the freedom and rights of the slaves/colored people as compared to those of the whites.

Though there is no specific rhyme scheme in the poem, there are still some instances of the poem having end rhymes with vowel sounds and some cases of assonance. The use of rhyme in the poem is the attempts of the poet  to lay emphasis on the song sung by the "caged bird". The caged bird, as a representation of the blacks, sings a song of imprisonment, devoid of liberty and freedom and made to suffer unlike the 'free bird'.

The repetition of the third stanza in the last stanza brings back the focus of the poem, giving it greater importance. Also, there are a lot of rhyming in these lines of the third and last stanzas.

with a fearful trill    

of things unknown    

but longed for still    

and his tune is heard    

on the distant hill    

for the caged bird

Thus, the correct answer is that the poet uses rhyme to emphasize the caged bird's song.