Answer:
D.
Explanation:
Mongol Empire was established under Genghis Khan in 1206. Genghis Khan was considered one of the powerful ruler who almost conquered the whole world. His territory expanded from Pacific Ocean in the east to the west to the shores of Persian Gulf.
During Mongol empire, Genghis Khan created the fastest mode of communication between the empires by greasing the wheels and sending posts. Khans developed a postal system by greasing the wheels which enabled them to deliver their posts within a day across the empire.
Therefore, the correct answer is option D.