The poet develop a theme of freedom, through the metaphor of singing.
Metaphor is figure of speech applied to symbolize something else.
Because the metaphor of singing develops the theme as the caged bird is unable to enjoy her freedom because her wings are clipped and her feet's are tried, so the freedom is symbolized by the caged bird song, the song is the true longingness for freedom and represents something great in life like freedom.
Hence the correct option is, through the metaphor of singing
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