Final answer:
The incident of hearing the old man's heartbeat provokes the narrator's decision to murder him in 'The Tell-Tale Heart' by Edgar Allan Poe.
Step-by-step explanation:
The incident of a new anxiety seizing the narrator in Edgar Allan Poe's 'The Tell-Tale Heart' provokes his decision to murder the old man because he is afraid someone will hear the old man's heartbeat, thwarting his chance to commit the murder. The narrator is fearful that the sound will be heard by a neighbor, revealing his crime. This fear pushes the narrator to carry out the murder immediately in order to prevent anyone from discovering his dark secret.