Japan had a highly developed culture and society prior to 1500. It was able to borrow from Chinese culture while maintaining its own very distinct societal values. Japan was isolated from mainland Asia by geography.

Which of these best explains why the Chinese and Mongols were unable to occupy Japan?

One of the main reasons was a typhoon that struck the Mongol fleet and sunk almost all of the ships. The Mongols would return a couple of decades later, only for the same fate to occur. The Japanese referred to this phenomenon as the "Kamikaze" or Divine Wind. Another reason that the Mongols would have been unable to occupy Japan, at least the second time around, was that the Japanese had prepared for their return and built a lot of shore defenses. The samurai and the warrior oriented society of Japan, as compared to China would have also aided in defending them from an invasion. The warrior tradition of Japanese surrender, and the belief held by the Samurai that death was favorable to the dishonor of surrender meant that they would fight to the death - the Mongols, though dedicated and feirce conquerors, would not.