Question

which of these is a poetic device Walt Whitman often uses to emphasize certain ideas in his poems

Answer 1

 Well poetic devices Walt Whitman used was imagery. He enjoyed imagery in certain ideas in his poem because he could express them better. 
 
Hope this helps. 

Answer 2

Walt Whitman often uses alliteration to emphasize certain ideas in his poems. Alliteration in literature is understood as the repetition of identical initial consonant sounds in successive or closely associated syllables within a group of words, even those which are spelled differently. Besides, alliteration is also called head rhyme or initial rhyme.

An example of alliteration in one of Walt Whitman's poem may be the repetition 11 times of the word singing in "I Hear America Singing."