B is the closest to the correct answer.
Rome had, in many ways, gotten to be too big. Governorships were instituted that diluted the centralized power of Rome and these Governorships were often sources of great corruption. Eventually, the Germanic people had enough and invaded.
Additional causes included a tax system that eventually diluted the value of currency, an empire that had gotten too large, and an economic system built on the back of slaves that couldn't figure out what to do next with the slaves.
