Answer: C. large earthen mounds built for religious purposes.
The Mississippian culture developed in the Eastern United States around 900-1450 CE, along the river valleys. The culture had several regional variants including the Middle Mississippians, the South Appalachian Mississippians, the Plaquemine Mississippians, and the Caddoan Mississippians. This culture is particularly known for the gigantic earth mounds that they built. These served as burial and ceremonial places.