D. the Union honoring tribal treaties signed with the Confederacy
Explanation:
In The tribal treaties, the confederacy promised to not disturbed the Native's land in exchange for their help during the Civil War.
After the civil war ended, the Union decided no to honor these treaties and forced a lot of tribes to migrated out of their ancestral land.
The union wanted to used the land to provide enough space for railroads development. They wanted a fast method of distribution to deliver products from southern states to mid-norther states.
