The correct answer is A) to acquire new territory.
The United States' justification to go to war with Plains tribes was to gain more territory.
Under the idea of Manifested Destiny, President Jefferson allowed the occupation of the new territories of the West. The Clark and Lewis expedition had shown that the territory was vast and in need of occupation by white settlers to make the land productive. So conflicts were permanent between Native American Indian tribes and white settlers since the colonial times. Things got worse with decisions such as the Dewes Act of 1887, that ended the reservation system. Among the Plain tribes affected were the Kiowa, Comanche, and Cheyenne.