Answer:
d. to serve as a pivot point for the poem.
Step-by-step explanation:
In "The Philosophy of Composition," Poe describes how he was able to produce his poem "The Raven," one of his most famous and popular works. One of the things that he details is the fact that the refrain of the poem is meant to serve as a pivot point for the poem. This means that the refrain is held as a constant throughout the poem, and the other elements are built around it, providing a sense of regularity.