The appearance of the raven in the passage from "The Raven" by Edgar Allan Poe propels the narrative action of the story in the following way: B: It introduces a new character by which readers are able to gauge the speaker's deteriorating mental state.
The raven serves as a symbol and a messenger in the poem, representing both the speaker's past and the potential loss of Lenore, as well as the encroachment of death.
The raven's presence and actions throughout the poem help to illustrate the speaker's mental decline and his inability to cope with the death of his loved one.
Thus, the raven's appearance serves to heighten the tension and drama of the story, as well as to reveal the speaker's unstable emotional state.