Answer:
Found this off of google, "Constantine now became the Western Roman emperor. He soon used his power to address the status of Christians, issuing the Edict of Milan in 313. This proclamation legalized Christianity and allowed for freedom of worship throughout the empire. ... In 324, Constantine defeated Licinius and took control of a reunited empire."
So, the answer to your question is D)He made Christianity the sole religion, to unite everyone.
Hope this helps have a great day and stay safe! :) :D :3