The answer is: B) a type of bird.
In the passage from " The Song of the Cardinal," by Gene Stratton-Porter, the narrator describes two kinds of songbirds. On the one hand, the green warbles, which are insectivorous birds that sing softly with constant changing notes. On the other hand, the vireos are American songbirds depicted in the excerpt as having red eyes and located below the tree tops, under the warblers.