The correct response is China.
While the Mongolian Empire did extend into parts of China, it was unable to conquer the entirety of the region due to several factors. The vast and diverse geography of China, including its mountainous terrains and dense population centers, posed significant challenges for Mongol expansion. Additionally, the strong fortifications and organized resistance offered by Chinese dynasties, such as the Southern Song dynasty, prevented the Mongols from establishing full control over the entire country.
Thus, while the Mongols made significant incursions into China, they were not able to conquer the entire nation due to these geographical and political complexities.