The correct answer is B. Because a one-word refrain is less difficult than a long refrain.
The refrain in Poe's 'The Raven' consists of one word only because of its brevity (shortness). This is what Poe wrote in his The Philosophy of Composition (a text about how he went about writing this poem):
Since its application was to be repeatedly varied it was clear that the refrain itself must be brief [...] This led me at once to a single word as the best refrain.
