Answer:
She tricks Framton into believing a falsehood that sends him into a panic.
Step-by-step explanation:
The Open Window story begins with the arrival of Nuttel Framton, a hypochondriac, to the country, looking for someone to host him in their home, as he needed a more quieter place to be more relaxed and deal with his nerve issues. The plot develops when he arrives at the home of his new possible hosts: the Sappetons, and is received by Mrs. Sappleton's niece, Vera, a fifteen-year-old girl.
While Nuttle waits for Mrs. Sappleton to come out, Vera starts explaining Framton why the french window is open, she says that it remains open because Mrs. Sappleton believes that one day his husband, children and the dog's family (who had died in a tragic hunting accident years ago) would return home someday and re-enter the house. This is just a lie, of course, but the girl was so convincing that Nuttle believes it.
Once Mrs. Sappleton meets him, the hunting party appears walking toward the house. Mrs. Sappleton looks at them with excitement but the mischievous girl looks at them horrified as if she was looking at actual ghosts. Nuttle, horrified and scared, flees the house immediately.
Therefore, we can conclude that Vera advances the plot by tricking Framton into believing a falsehood that sends him into a panic.