The correct analysis of the literary device used in the phrase "The only other sound’s the sweep / Of easy wind and downy flake" highlights its imagery as a means to create a serene and peaceful mood. This choice of words paints a calming picture of a gentle winter scene, where the soft, effortless movement of the wind and the lightness of the snowflakes contribute to a tranquil atmosphere. Thus, the most suitable response would be:
"The imagery of 'sweep' and 'downy flake' creates a serene and peaceful mood, enhancing the calm yet contemplative atmosphere of the poem."
This imagery effectively draws the reader into a moment of quiet reflection, emphasizing the beauty and stillness of the setting.