The correct answer is C: large earthen mounds built for religious purpose.
The Mississippian culture, which thrived in the southeastern United States from around 800 CE to 1600 CE, is well-known for its construction of large earthen mounds, which served various purposes including religious ceremonies, burials, and as sites for elite residences and civic activities. These mounds, such as Cahokia's Monk's Mound, are significant archaeological evidence of the social, political, and religious complexity of Mississippian society.