The correct option is D
The Roman Empire succumbed to barbarian invasions mainly due to the loss of the traditional Roman civic virtues by its citizens. These would have become weak, delegating the task of defending the Empire to barbarian mercenaries who became so numerous and rooted in the Empire and its structures that they were able to take it at last. The Romans, after the fall of the Republic had been becoming progressively "effeminate", unwilling to live a military life, This would have led to the progressive abandonment of their freedoms in favor of the tyranny of the Caesars, and would have led to degeneration of the Roman army and the Praetorian Guard.
