The correct answer is C. Her familiarity allows her to invent an otherworldly explanation for events she can easily predict; Nuttel’s unfamiliarity makes him believe this as truth.
Vera's knowledge of her aunt's tragic past and the associated circumstances enables her to craft an elaborate story that plays on Framton Nuttel's ignorance of the family's situation. She uses this to create a suspenseful atmosphere that ultimately leads to Nuttel's terrified reaction when he sees the returning family members—effectively making him believe in her invented narrative.