Final answer:
The geographic isolation of Mesopotamia, due to a lack of natural building materials, constrained the early Sumerians' construction choices, leading them to make extensive use of mud bricks.
Step-by-step explanation:
The factor that limited the choices of construction system available to the early people of Sumeria in Iraq was the geographic isolation of the region (Choice C). Because Mesopotamia lacked natural resources like timber, precious metals, and stone, the Sumerians had minimal choices for building materials. This limitation led them to utilize what was available, such as mud bricks made from the rich soil and clay found in the valley between the Tigris and Euphrates rivers. The mud bricks were essential for constructing their massive ziggurats, which still stand today. Moreover, the area's rich soil for agriculture (Choice D) was not a limiting factor but instead contributed positively by supporting extensive farming, which in turn supported the civilization's development through surplus food production.