East China Sea served as an eastern border for the Mongol Empire. Option D is correct.
The Mongol Empire existed during the 13th and 14th centuries and constituted the largest contiguous land empire in history.
The Mongol Empire expanded from Eastern Europe and parts of Central Europe to the Sea of Japan, spanning northwards into Siberia, eastwards and southwards into the Indian subcontinent, Indochina and the Iranian Plateau; and westwards as far as the Levant and the Carpathian Mountains.