Answer:
A. Invasion by Nomadic warriors.
Explanation: The Mandate of Heaven was a doctrine use to justify an Emperors rule. According to beliefs, Heaven, imposed the doctrine on a just ruler. But if there were to be droughts, plagues or if the ruler would be overthrown; it was viewed as heaven being displeased with the emperor and the mandate had been withdrawn. In this case, an invasion can be seen as a sign of punishment.