Poe's use of repetition and dashes in the quote creates a nervous, suspenseful mood and tone. The repetition of "very, very, dreadfully nervous" emphasizes the narrator's heightened emotional state and instability. The dashes contribute to a sense of fragmentation and chaos in the narrator's thoughts, reflecting their anxiety and urgency. This combination effectively conveys the tension and unease inherent in the story.
Therefore, the correct answer is:
it gives a nervous, suspenseful mood and tone.