The correct answer is a) Native Americans had to renegotiate treaties with the U.S. government after the Civil War because many of them had fought for the Confederate Army, so they were percieved as an enemy of the nation.
Among the nations or tribes that had traeties with the Confederacy were: Chactaw, Chickasaw, Osage, Seneca, Shawnee and Quapaw; while the Cherokee, Muscogee and Seminole fought for both sides.
After the war, a comission was formed (Southern Traety Commission) that summoned the leaders of the tribes to sign new treaties ith the government, this was not optional, it was mandatory.