Answer:
c. Hun invasions that triggered a mass migration.
Step-by-step explanation:
The first major blow to the Roman Empire comes from Huns a nomadic tribe but in an indirect way. When the Huns began their westward expansion it forced the other tribes of Gothic people, Scythians, Vandals, and other tribes to infiltrate into the western boundaries of the Roman empire. This led to the disintegration of the Roman empire and led away to the rise of other European empires.