The answer is D (the person who answered above me is incorrect). The inca were organized to acheive complex projects.
From conquered populations, the Incas demanded a tax of labor. This tax could be paid with military service, construction work, or agriculture work. It was this organized and intensive system of labor that allowed the Incas to complete massive construction projects. The Incas built on the engineering accomplishments of the mother cultures that preceded them in the Andes region.
The citadel is important, because it has a great archaeological value and a variety of ecosystems. Machu Picchu symbolizes the excellent technical skill, and productivity of the Inca Empire in its apogee.
