Answer:
The correct answer is C. They sought better land and more livable climate.
Step-by-step explanation:
The Mongols were nomadic pastoralists that travelled with their flocks of sheep, horses and cattle. In order to keep their livelihood, they had to seek better lands with pastures and grassland to feed their cattle. Since most of Mongolia was semiarid steppe, dry grassland and prairie, the Mongols had to move west and south to find better lands and a more livable climate.