The observation that indicates that Aristotle’s geocentric model was flawed is:
Objects in the sky appear in different areas of the sky each night.
This observation suggests that the positions of celestial objects change over time, which is inconsistent with a strictly geocentric model where all celestial bodies should maintain fixed positions relative to the Earth. The changing positions imply the need for a more complex model of the cosmos, eventually leading to the heliocentric model proposed by Copernicus.