The correct answer is - C. the Chinese.
The biggest achievement of Genghis Khan's nephew, Kublai Khan, was that he managed to defeat the Song dynasty in China. After he managed to defeat the Song dynasty, Kublai Khan managed to organize an unified China for the first time, where the tolerance was on a high level, and it was the basis from where he was able to achieve other things. Kublai Khan also moved the capital of the Mongol Empire in Beijing.
The unified China was in place until Kublai khan was alive, but son after his death, the Mongol Empire started to decline and the Chinese pushed back the Mongols into the northern steppes.