Which was the main way the Incas modified their environment to grow more food?

"The Terraced fields" would be the correct solution.

Explanation:

The Incan agriculture has been the collection of practices rendered on its territory either by Peruvian Inca community throughout the Andes rock formation characterized mostly by rocky terrain.The terraced fields really are the main way this same Inca farmers was using to produce vegetables throughout the mountains, they built an architectural style to accommodate the rough culture by promoting horizontal surface lands or verandas that encourage the cultivation and irrigation of the crops.

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