Answer:
Correct answer is The Mongols struggled to govern such an enormous empire.
Step-by-step explanation:
First option is correct as Mongols created one of the greatest empires ever. The problem was that it was hard to control the Empire, especially because there were frequent rebellions, wars for succession and similar things.
Second option is not correct, as Mongolian leaders were collecting taxes on the territories they controlled rather easily.
Third is not correct as they were not adopting themselves to it.
Fourth is also not correct, as they would established good links (especially trade links) in most of the regions they controlled.