The correct answer is - carving terraced farmlands.
The Incas revolutionized the agriculture in South America by starting to use the carved terraces for production. With this technique, the Incas managed to maximize the arable land area, and to perform farming on places where naturally will be impossible. This method is widely used nowadays, and since the amount of arable land is reducing in the lowlands, it is of great benefit for food production in places that are not suitable for building.
