The Answer is C) Authority.
The word dominion means "to have the right for controlling or ruling something or somebody". In this case, there is a control over territory.
We could rephrase the sentence the following way to have a clear idea of what "held dominion" means:
In alliance witht the Comanches, they [the Kiowa] held control of the southern Plains for a hundred years.
Finally, if they held control of the southern Plains, meant that they had the authority to do it.
