Answer:
b. The death of Genghis Khan.
Explanation:
The great Mongol conqueror Genghis Khan died on August 18, 1227 AD. He had built an empire that extended from the Sea of Japan to the Caspian Sea. He conquered more land than any other previous great conqueror in history. His hordes invaded Russia in 1223 AD and were preparing for an all-out assault on Europe, but the Mongol invasion of European lands in the early 13th century was temporarily halted by his death. His heirs and the top commanders of his armies had to go back to the Mongolia for the election of the new great khan. However, Mongol hordes invaded Europe again in the following decade.