Who were the mongols and what did the do under their leader genghis khan and his successors

Mongols were a nomadic people stemming out of Asia who conquered a big portion of the living and known world at the time (they were active during the middle ages; more specifically during the 12-13th century). They arrived all the way to Bulgaria and modern-day Hungary for example. They also conquered China, the Middle East and parts of the Indian subcontinent. 

The answer should be b