Which stanza structure does emily dickinson use in this excerpt from “hope is the thing with feathers”?

hope is the thing with feathers
that perches in the soul,
and sings the tune without the words,
and never stops at all,

and sweetest in the gale is heard;
and sore must be the storm
that could abash the little bird
that kept so many warm.

a.
couplet
b.
quatrain
c.
sestet
d.
octave