Which phrase from john keats's "to autumn" contributes to a mood of serene stillness?

a.
"sitting careless on a granary floor, /thy hair soft-lifted by the winnowing wind"

b.
"close bosom friend of the maturing sun"

c.
"in a wailful choir the small gnats mourn . . borne aloft / or sinking as the light wind lives or dies"

d.
"summer has o'er-brimmed their clammy cells"