Which of the following is not a reason for the decline of the mongolian empire

While you didn't provide a lost of possible answers, the correct answer to this question would be, based on the following questions:

AThe Mongolians lacked an orderly succession policy. 
BThe Chinese resented being ruled by a foreigner. 
CEuropean imperialism in Asia overcame Mongolian forces. 
DKublai's two failed invasions of Japan emptied the treasury and weakened his forces.

C - European imperialism in Asia overcame Mongolian forces. This is the correct answer because EU imperialism came a few centuries later.