Final answer:
The Atacama Desert served as the geographic feature marking the southern boundary of the Inca Empire.
Step-by-step explanation:
The geographic feature that marked the southern border of the Inca Empire was the Atacama Desert. The Inca Empire flourished in South America, specifically in the Andes region, which includes modern-day Ecuador, Peru, and Bolivia. Surrounding geographic features included the massive Amazon River system and the extensive Andes Mountain chain. However, the harsh and arid conditions of the Atacama, known as the hottest desert on earth, served as a natural boundary to the southern expansion of the Inca Empire.