The correct answers are B to protect Native Americans from settlers and to civilize the Native Americans C to take land legally and to enrich the United States.
Indeed, Indian Tribes had been subject to the violence of either the Army or private militias under the service of individual wealthy settlers or armed settler communities that forced them to sell their lands under threat of extreme violence. The federal government wanted to make sure that Native American Tribes would be appropriately represented in their negotiations for legal treaties and backed by the government against the intimidation tactics of private individuals.
Of course, it was not all altruism; the approach also allowed the federal government to keep the peace, provide a legal frame for the acquisition of Native lands and thus enrich the United States when such lands contained valuable minerals or animals.