Answer:
Explanation:The event that most directly led to the loss of Sioux land in South Dakota in 1877 was the discovery of gold in the Black Hills (option a).
When gold was discovered in the Black Hills, it triggered a gold rush in the area, attracting a large number of settlers and miners. This influx of people violated the Treaty of Fort Laramie of 1868, which had designated the Black Hills as Sioux territory. Despite this, the U.S. government did not enforce the treaty and instead encouraged settlers to enter the area for gold mining.
The Sioux, who considered the Black Hills sacred and vital to their way of life, resisted the encroachment on their land. However, the U.S. government responded with military force and attempted to remove the Sioux from the area. This ultimately led to the Great Sioux War of 1876-1877, including the famous Battle of Little Bighorn.
The U.S. government's military campaign against the Sioux resulted in their defeat and the subsequent loss of their land in South Dakota. The government then confiscated the Black Hills and opened them up for settlement, despite the Sioux's continued protests.
Overall, it was the discovery of gold in the Black Hills and the subsequent influx of settlers that directly led to the loss of Sioux land in South Dakota in 1877.