Which conflict prompted the U.S. government to open Oklahoma lands to settlers?

The correct choice is the removal of Native Americans, a pivotal event that set the stage for the opening of Oklahoma lands to settlers. During the 1830s and 1840s, the U.S. government implemented policies that forcibly relocated numerous Native American tribes from their ancestral lands in the East to designated "Indian Territory," which is modern-day Oklahoma. This removal was largely driven by the desire for land for agricultural expansion and settlement by white settlers.

As tribes such as the Cherokee, Chickasaw, Choctaw, Creek, and Seminole were displaced, vast areas of land became available for settlement. The federal government, looking to encourage settlement in the West, organized land runs and various land policies that allowed settlers to claim these once-occupied lands. The Homestead Act and subsequent land rushes in the late 19th century were direct results of this earlier removal policy, creating significant opportunities for non-Indigenous individuals to settle in Oklahoma.

In contrast, the Civil War, the Spanish-American War, and World War I did not directly prompt the opening of Oklahoma lands for settlers, as these conflicts were more about national issues, international relations, or military engagement rather than land policy and settlement strategies within the United States.