Answer:
B. To protect against barbarian invasions.
Explanation:
During the test reigns of Roman Empire as the largest and most powerful state, boasting of higher population, and having vast land territories which span across Europe and North of Africa, the Romans are not left out from the preyin eyes of the barbarians who not only seek for better life in the Roman Empire state, but also seek to take over from the wealth and technological advancement of Roman Empire.
Hence, the Roman Empire leaders, in their bid to secure the state and the citizens against the Barbarian invasion, most Roman soldiers were stationed on the borders of the Empire, specifically, TO PROTECT AGAINST BARBARIAN INVASION.