Final answer:
The early people of Sumeria in Iraq were limited in their construction system choices by the marshy terrain of their region which lacked natural resources like wood and stone, leading them to utilize mud bricks as a primary building material.
Step-by-step explanation:
The factor that limited the choices of construction systems available to the early people of Sumeria in Iraq was c. Marshy terrain. The region they inhabited lacked significant natural resources like stone or timber, making alternative building materials necessary for construction. Due to the scarcity of wood and minerals, the Mesopotamians utilized what they had in abundance: mud from the fertile valleys along the rivers. They crafted mud bricks for their structures, such as the massive ziggurats, which have withstood thousands of years. Furthermore, the absence of stone also meant that monumental architecture in Mesopotamia was developed with the use of sun-baked bricks and clay, which necessitated a different construction approach compared to civilizations with access to wood or stone.