The correct answer is - D. internal strife.
The Western Roman Empire had its biggest enemy in its own country, and that was the lack of unity, and constant disagreements between the elite about key questions. Also, in order to get to more power, the Roman elite was taking all actions necessary, set ups, economic destruction of the opponent, murder, and they seemed to forgot the well-being of the empire while trying to get as much as they possible can for their own personal interest.
This situation was used by some of the stronger nations, like the Huns, Goths, Vandals, that little by little managed to destroy the Western Roman Empire, and finally take down the city of Rome itself.
