I think "A" is a good answer.
Read the following:
Charlemagne chose officials, called counts, to help assist him with the empire. ... With Charlemagne getting near the end of his life, the empire started to experience a downfall. Eventually, the Carolingian Empire fell, even more so after Charlemagne's death. Like most empires, this one fell after the rule of the emperor.
Thus, the downfall of his empire started during his rule when he was near the end of his life "the empire started to experience a downfall".