The correct answer is option A. "Sioux indian attacks on stagecoach lines". The Sand Creek Massacre was a massacre of Cheyenne and Arapaho people by the U.S. Army during the American Indian Wars, which resulted in 70 to 500 Indian people dying in Colorado. This terrible event is historically attributable to the constant Sioux Indian attacks on stagecoach lines in Colorado and other parts of the United States territory, that provoked US military forces mobilization into indian territories.
