Why did general sheridan choose to attack native american tribes in the winter?

Because in 1867, Ulysses S. Grant charged Sheridan with pacifying the Great Plains, where warfare with Native Americans was wreaking havoc.  In an effort to force the Plains people onto reservations, Sheridan used the same tactics he used in the Shenandoah Valley:  he attacked several tribes in their winter quarters, and he promoted the widespread slaughter of American bison, their primary source of food.  

Tribes were most vulnerable in freezing temperatures.

the mongolian invasion of china and central asia resulted in a disruption of the silk road as the chinese garrisons lost support from the chinese kingdoms which fell under the mongols. however, as the mongols established a formal dynasty in china, the successors of genghis khan restored the garrisons along the silk road allowing the safe travel caravans again.

d is the best answer for this question.