The correct answer is c. large earthen mounds built for religious purposes.
The Mississippian culture, which flourished in the southeastern United States from approximately 800 CE to 1600 CE, is well-known for its construction of large earthen mounds, which were often used for religious, ceremonial, and political purposes. These mounds, such as Cahokia's Monks Mound, provide significant insight into the societal structure, religious practices, and community organization of the Mississippians.