Final answer:
Option (b), The Springer Amendment was a rider attached to the Indian Appropriations Act of 1889, leading to the Oklahoma Land Rush by opening previously Native American designated land to white settlers.
Step-by-step explanation:
The rider attached to the Indian Appropriations Act of 1889 was the Springer Amendment. This amendment essentially led to the opening of the "Unassigned Lands" in Oklahoma to white settlers. It was named after Illinois Congressman William M. Springer, who was responsible for the incorporation of the amendment into the appropriations bill.
The Springer Amendment paved the way for the famous Oklahoma Land Rush, where settlers could claim parcels of land previously considered as part of the public domain and designated for Native American tribes. It reflects the United States government's policy at that time of reallocating tribal lands, often without regard for the existing inhabitants, and represents a significant event in the history of westward expansion.