Final answer:
The speaker's mention of 'Elves' in Robert Frost's 'Mending Wall' is an allusion, which implies a mystical or unexplained reason for the natural decay of the wall that goes beyond the mundane task of repairing it.
Step-by-step explanation:
The reference to elves in the excerpt from Robert Frost's Mending Wall is best described as a allusion. An allusion is an indirect or implied reference to something, in this case, mythical creatures. The speaker brings up elves to highlight the unseen forces that contribute to the natural decay of the wall, suggesting a mystical or unexplained reason for why the wall keeps needing repairs. However, the speaker also prefers that the neighbor considers this idea on his own without the speaker explicitly stating it. The mention of mythical creatures introduces a fantastical element to a mundane task, creating a contrast between the mythical and the realistic.