The correct answer is C) the Battle of Little Bighorn
The conflict that resulted in the defeat of the US Army was the Battle of Little Bighorn.
The Battle of Little Bighorn resulted in a major defeat of the Calvary of the United States. The Arapaho Tribes, the Lakota, and the Cheyenne fight the 7th Cavalry Regiment led by Colonel George Custer. The Native American Indians had a superior number of men and they practically destroyed the US soldiers and Colonel Custer was killed on the battlefield.