Answer:
Correct answer is Pax Romana occurred while Rome was an empire when wars were less frequent and the empire grew strong.
Explanation:
A is not correct as during the Republic were a lot of unrests. Slave revolts, Civil wars between Sula and Gaius Marius, Caesar and Pompey, Octavian and Marcus Aurelius, Punic wars... It was hard to establish peace.
B is correct as during Octavian Augustus reign Rome as an empire spread its boundaries and for a period of around 200 years led not so frequent wars, which were successful. Roman culture, economy and all other aspects of society were flourishing.
C is not correct because as we said during period of Republic there were many unrests.
D is not correct as Julius Caesar led many military campaigns, mostly in Gaul and Britain and a civil war against Pompey.