The correct answer is - 4. the decline of Rome.
The Roman Republic was already in bad shape when Octavian rose to power, but he is the one that had the infamous role of being the emperor under which Rome failed. The Roman Empire was exhausted, and badly damaged, both by the attacks of the Huns, Goths, and Vandals, and its internal struggles where everyone was only concerned about their own power and not the state of the empire. This marked the decline of Rome, and eventually its falling and occupation.