In "Dances with Wolves," Lieutenant John Dunbar is adopted into the Sioux tribe. After arriving at a remote frontier post, Dunbar gradually builds a relationship with the nearby Indigenous community, especially Kicking Bird, Wind In His Hair and Stands with a Fist.
The people eventually accept Dunbar, give him the name Dances with Wolves and treat him as one of their own. He learns their language and customs, marries Stands with a Fist, and chooses their community over returning to the U.S. Army. The film presents this adoption as the culmination of Dunbar's rejection of military expansion and his growing identification with Lakota life.
"Sioux" is the broad, commonly used answer in trivia, although the community depicted is more specifically Lakota, one of the major divisions of the Sioux people. That distinction can matter in historical discussion, but it does not make the stored answer wrong for this question. Another common mix-up is thinking Dunbar adopted Stands with a Fist; she was a white woman adopted by the tribe as a child, while Dunbar himself is later accepted into the community.