What two cultures are known as mound builders and why

The Adena and Hopewell cultures are known as mound builders. Archaeological research indicates the mounds of North America were built over a long period of time by very different types of societies, ranging from mobile hunter-gatherers to sedentary farmers. The prehistoric mounds had a wide variety of forms and fulfilled a range of functions. Many served as burial mounds, individual or collective funerary monuments. Others were temple mounds, platforms for religious structures. Burial mounds were especially common during the Middle Woodland period (c.100 BC–AD 400), while temple mounds predominated during the Mississippian period (after AD 1000).

Im pretty sure the answer is d.) is more densely populated.