Answer:
the heavy tectonic activity in the area
Step-by-step explanation:
"The heavy tectonic activity in the area" had the greatest impact on South America's physical geography. This is true because the heavy activity of the tectonic in the area such as plate motions resulted in the formation of mountains and oceans in South America.
For example, the formation of the massive Andes Mountains and the formation of various volcanoes resulted from heavy activity of the tectonic in the area.