What is the word for a poem's basic rhythmic unit?

With reference to the form of the selection,

A. the form is ternary (A B A) B. the voices sing alone at the very beginning C. voice stanzas alternate with harp solos D. phrases are avoided in the vocal solos

Most of Ives' music was written in the ________ century.

A. late nineteenth B. recent twentieth C. early twentieth D. mid-twentieth