The correct answer is Vera's tendency for making up outlandish stories.
From the story "The Open Window" by H.H Munro (Saki), it is evident that Vera tells Framton a story about her aunt, but to the reader, the means of this is nothing more than to inform their visitor about the family's past - as if to warn Framton on his involvement with his visit -. It is only when a terrified Framton bolts away from the house and the rest of the family members are presented safe and sound, that the reader realises Vera's story was only a playful trick, creating surprise - this information however, is not explicitly explained to the reader.
As for the rest of the options, unlike the correct answer, they are all information explicitly presented in the story:
- The presumed Mrs. Stapleton's insanity at the beggining of Vera's tale.
- Framton explains his illness after Mrs. Stapleton arrives to the scene apologizing.
- Framton's fear of dogs is explained by Mr. Stapleton at the last paragraph, just after his arrival.