Final answer:
The most likely cause of conflict in the passage 'The Open Window' is the protagonist's mental illness, leading to her misinterpretation of her confinement.
Step-by-step explanation:
The most likely cause of conflict in the passage 'The Open Window' is the protagonist's mental illness. The narrator's unreliable perception and interpretation of her confinement in a child's former playroom creates conflict within herself. This can be seen through her misinterpretation of the details in the room, such as the bars on the windows, which she associates with confinement.