“The Open Window” is a short story written by Saki and in this case the author provides evidence that Mrs Sappleton knew what her niece was doing and her ability for storytelling.
Option A) is the correct one “She says that Framton acts like he has seen a ghost.” since it explains why Framton left the place running as it coincides with the story told by the niece about ghosts.
Option B) is incorrect since there is no evidence that Mrs Sappleton believed the story about dogs told by the niece when Framton ran away.
Option C) is also incorrect since she never tells her husband that Vera scared Framton.
Option D) is also incorrect since even though she asks Framton if her niece has been amusing him, she does not say it to let the reader know she knows about her niece.