Final answer:
The statement is true. Cahokia's decreased mound building after 1200 suggests a societal decline of the Mississippian culture. By 1250, Cahokia entered a period of decline and was ultimately abandoned.
Step-by-step explanation:
The statement is True. Decreased mound building after 1200, during the so-called Moorehead Phase, suggests a societal decline of Cahokia. Cahokia was once a thriving Mississippian culture that created large urban settlements in the Mississippi River valley.
However, by 1250, Cahokia entered a period of decline and was ultimately abandoned, marking the decline of the Mississippian tradition itself.