The correct answer is Vera's tendency for making up outlandish stories.
From the story<em> "The Open Window"</em> 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.
