The correct response is: It seems to darken into a deeper tragedy.
When the aunt enters the room, there is a significant shift in the atmosphere that introduces a sense of foreboding regarding the story's unfolding tragic elements. This moment contrasts with the relative lightness of the conversation between Framton Nuttel and the niece, suggesting that the arrival of the aunt brings with it a darker undertone related to the story's theme of loss and sorrow. The mention of the aunt’s tragic backstory concerning the open window and her late husband and brothers prepares the reader for the emotional weight that will follow.