Final answer:
Native American civilizations faced threats from diseases like smallpox introduced by Europeans and the near-extinction of buffalo herds, both of which dramatically affected their populations and way of life.
Step-by-step explanation:
Native American civilizations were threatened by option C. both diseases introduced by white settlers and the disappearance of the buffalo herds. These factors were significant in the decline of their populations and way of life. The diseases such as smallpox, diphtheria, and measles brought by Europeans had a catastrophic impact on Native Americans, who had no natural immunity to these illnesses. Additionally, the buffalo, which was crucial to the Native American way of life, especially for the Plains tribes, was nearly hunted to extinction by white settlers, disrupting the Native Americans' source of food, clothing, and tools.