The correct answer is b. Her familiarity allows her to invent an otherworldly explanation for events she can easily predict; Nuttel's unfamiliarity makes him believe this as truth.
In "The Open Window," Vera's knowledge of the setting and the circumstances around her family's absence allows her to craft a convincing and elaborate story that exploits Nuttel's ignorance. She manipulates the situation, causing Nuttel to become increasingly frightened and confused, ultimately leading to the story's climax. His unfamiliarity with the local context and her family's situation makes it easier for her to deceive him effectively.