The Edit of Milan contributed to the growth and longevity of the Roman Empire because it made it safe for the larger Christian community to practice their religion.
EXPLANATION:
The Edict of Milan was a letter that the Roman Emperors "Licinius and Constantine", wrote and signed on February 313 AD. In this letter the emperors proclaimed religious tolerance for the Roman Empire, mainly destined to make the large amount of Christians that the Empire had, feel safe.
Ostrogoths trace their origin to another tribe, the Greuthungi, who were a semi-legendary tribe ( so we're not sure they existed) who dwelled next to the Black Sea- so answer C. is the correct one.
