The mound builders were american indian groups who a. built irrigation systems for farming. b. developed sophisticated art. c. stayed in their own communities. d. flourished in the eastern river valleys.

The correct answer is D. The Mound Builders were American Indian groups who flourished in the eastern river valleys.

Explanation:

The culture of the mounds is a pre-Columbian archaeological culture that developed in the south and southeast of the current United States. Mound Builders were the carriers of this culture, which probably belonged to different ethnic and linguistic groups. The highest concentrations of burial mounds occur in the state of Ohio, where more than 10,000 have been located. One of the most representative sites of mound cultures is Cahokia, in the Mississippi Valley.