Answer: The correct answer is C. Constantine.
Explanation: Constantine the Great, the Roman Emperor, is known for issuing the Edict of Milan in 313 AD, which granted religious tolerance to Christians. He played a significant role in the Christianization of the Roman Empire and, though not declaring it the official religion, he took measures that facilitated the rise of Christianity, such as legalizing and promoting the faith. It was Emperor Theodosius I who later declared Christianity the official religion of the Roman Empire in 380 AD through the Edict of Thessalonica.