Final answer:
Julius Caesar played a key role in the Great Roman Civil War, which took place between 49 BCE and 45 BCE. Therefore, the correct option is D) Great Roman Civil War.
Step-by-step explanation:
Julius Caesar played a key role in the Great Roman Civil War. This civil war took place between 49 BCE and 45 BCE and was a power struggle between Caesar and Pompey the Great. Caesar eventually emerged as the victor, establishing himself as a dictator and paving the way for the transition from the Roman Republic to the Roman Empire.