Answer:
C. The rules that determine a poem's structure.
Step-by-step explanation:
In literature, a work of art consists of form and content. A poem's content would be its essence, the fundamental idea and its development.
A poem's form, on the other hand, refers to the way this original idea is put into words.
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 the way these rules are executed, they result in different poetic forms (sonnets, ballads, odes...).