The correct answer is:
D. Settlers violated the treaty by invading Sioux territory in order to claim gold discovered in the Black Hills, leading to the Great Sioux War.
The Treaty of Fort Laramie (1868) recognized the Black Hills as part of the Sioux territory. However, when gold was discovered in the Black Hills in the mid-1870s, prospectors and settlers began to invade the area, leading to tensions and conflicts that culminated in the Great Sioux War of 1876-1877.