The correct answer is D) understanding the effect of places and regions on the people living in them.
In South America, some farmers build farms on hilltops because of the mountainous landscape. One of the Six Essential Elements of Geography that this most represent is "understanding the effect of places and regions on the people living in them."
The six essential elements of Geography are the world in spatial terms, places and regions, physical systems, human systems, environment and society, and uses of geography.
In this case, South American Indian civilizations such as the Incas had to find ways to overcome geographical difficulties because the region they lived in was mountainous. Indeed, they were great engineers that built homes at the top of the mountains and developed agriculture techniques that allowed to grow crops at the top of the mountains.