Final answer:
The repeated word that serves to unify the poem 'I Hear America Singing' is 'singing'. This repetition creates a sense of unity and celebration in the poem.
Step-by-step explanation:
The repeated word that serves to unify the poem 'I Hear America Singing' is the word 'singing'.
Throughout the poem, the word 'singing' is used repeatedly to describe the various carols that the speaker hears.
This repetition creates a sense of unity and celebration in the poem, as it emphasizes the diverse voices of the American people coming together in song.