The best answer to how Julius Caesar affected the fall of the Roman Republic is:
C. By establishing a dictatorship while Cincinnatus used his power as dictator to protect the republic, Caesar used this power to replace it with an empire.
This option accurately reflects the shift from a republic to a more centralized autocratic rule under Caesar, contrasting his leadership with that of Cincinnatus, who returned power to the Senate and upheld the republican system. Caesar's actions ultimately contributed to the end of the Roman Republic and the rise of the Roman Empire.