He incorporates it to reflect a blues musical timing.
A signature of Hughes's poetic style is his meter. It reflects the rhythms of Harlem's jazz and blues during his time period, as well as black folk rhythms. Hughes's poetry is reflective of the African American culture in, not only its content, but also its rhythms.
The other two options are simply incorrect. He did not writing jingles for depression medication has nothing to do with how he incorporates meter. While the first does indicate the blues sound, it's not completely correct.