Answer:
C. The rules that determine a poem's structure.
Explanation:
In literature, a work of art consists of form and content. A poem's <em>content</em> would be its essence, the fundamental idea and its development.
A poem's <em>form</em>, on the other hand, refers to <em>the way this original idea is put into words. </em>
Traditionally, a poetic thought must abide to certain formal rules (meter, rhythm, rhyme scheme), thereby distinguishing the thoroughly defined laws of poetic creation from the significantly less restricted prose.
Depending on <em>the way these rules are executed</em>, they result in different poetic forms (sonnets, ballads, odes...).