Final answer:
The sight of the old man's glassy eye fills the narrator with revulsion and provokes his decision to murder him.
Step-by-step explanation:
The incident of the narrator gazing upon the old man's open eye in the excerpt from 'The Tell-Tale Heart' provokes the narrator's decision to murder the old man because the sight of the glassy eye fills the narrator with revulsion. The grotesque image triggers an intense emotional response in the narrator, leading to an overwhelming feeling of anger and fury. This drives the narrator to commit the heinous act of murder.
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