Final answer:
The buffalo was the main food and life source for the Plains Indians, which was hunted to near extinction primarily by white settlers for sport and to clear land for agriculture.
Step-by-step explanation:
The main food source and life source for the Plains Indians, which was hunted to near extinction by white settlers, was the buffalo (A). The buffalo was vital to the Plains Indians' way of life, providing not only food but also hides for shelter, clothing, and other necessities. When the Spanish introduced horses in the region, the local tribes adopted a more nomadic lifestyle, which made following and hunting the bison herds much easier. However, the arrival of white settlers brought about the near extermination of these herds, primarily for sport and to clear land for agriculture, drastically affecting the lifestyle of the native tribes and forcing them onto government-run reservations.