The answer is A: Constantine recognized the legality of Christianity. This was the biggest change in the Roman Empire to date. Constantine had a change of heart due to a dream he was seen by Jesus- and that is the result he became Christian. In addition, 'the great schism' was a barrier that Constantine and pope were separated (both which they agreed on) due to different ethnicities, languages & culture.
The great schism chart
Constantine's side
East side- middle easterners, Egyptians, turkey
Languages- turkish, Arabica
Pope's side
West side- Rome, Italy
Language- Latin