During which era did people begin to build mounds?

The Middle Woodland period

Explanation:

The Middle Woodland period (100 B.C. to 200 A.D.) was the first era of widespread mound construction in Mississippi. Middle Woodland peoples were primarily hunters and gatherers who occupied semipermanent or permanent settlements. Some mounds of this period were built to bury important members of local tribal groups.

d. the right to establish a one-house legislature