In the 1960s, the Native Americans were forced to go to be in the mainstream. They fought for their rights and created the Declaration of Indian Purpose. It took several years before their issue was addressed by the US. They were not the only minorities who fought for their rights and called for attention and opted for cultural pluralism and not cultural assimilation.
D. Allowing Texas into the Union would upset the balance of slave and free states in Congress. Though it soon became a state since Manifest Destiny was a very popular idea at the time.