The First Council of Nicea (325 AD) created the Nicene creed as a way to unify the fragmenting and diversifying beliefs in Christianity developing throughout the Byzantine empire. It establishes and explains the relationships between God, Jesus Christ, and the Holy Ghost. The other answers are incorrect because it was not crafted as legislation for political purposes, and was written to counter Arian Christianity, which questioned elements of the orthodox faith.

The First Council of Nicaea was convened by Emperor Constantine I in 325 CE. The main purpose of this council was to debate the nature of Jesus Christ in relation to his divinity according to Scripture. 

The council concluded that Jesus Christ was divine, co-eternal with God the Father as according to Christian doctrine.

Emperor Constantine I (also called Constantine the Great) supposedly had a vision during the Battle of Milvan Bridge in 312 CE in which he saw a cross of light hovering above the battlefield along with the words "in this sign, conquer".

The following year, the Edict of Milan granted tolerance to all religions, including Christianity. Christianity would later go on to become the state religion of the Roman Empire in 380 CE.

The Roman Empire had a long and sometimes contentious relationship with Judaism, however the Edict of Milan did not mention them, so Judaism would not be a good choice. Islam only became a popular and widely practiced religion after the fall of Rome, so that too would not be a good choice. Buddhism and Hinduism are ancient world religions, however the Roman Empire did not have any notable exposure to them, so they would not be good choices. Finally, the Edict of Milan specifically called for tolerance of Christianity, so that would be the best choice.

Theodosius was the first instill Christianity as the official religion after Constantine I had introduced it to the people of the Roman Empire. This question is all about time frame. If we remember that Constantine brought religion to Rome, then we also know that Theodosius I was the first ruler after to make it official. It would be tricky to confuse which Theodosius claimed the official religion. The other two options were no where near Constantine's rule of Rome. 

An ecumenical council is an assembly of Christian theologians who convene to debate and establish official church doctrine. The first ecumenical council was the Council of Nicaea in 325 CE. There were many other ecumenical councils throughout the remaining history of the Roman Empire, such as the Council of Chalcedon.

The Council of Nicaea was convened in 325 CE to debate issues surrounding Christianity. By this time, the nascent religion of Christianity had spread throughout the Roman world. Emperor Constantine converted to Christianity and, in doing so, made it inevitable that the religion would soon become the official religion of the Roman Empire. The Council of Nicaea was convened to debate theological issues, such as the nature of Christ’s divinity.

The Council of Nicaea was convened on the orders of Constantine I in 325 CE. The purpose of the Council of Nicaea was to debate various theological issues, such as the true nature of Christ’s divinity.

The Nicene Creed was established by the Council of Nicaea in 325 CE. The Council of Nicaea was the first ecunemnical council in the history of Christianity (there have since been countless). It was convened to debate issues of Christ’s divinity and to ensure universal agreement among the divergent theological schools of thought. The Nicene Creed is a profession of faith that states that Christ is the Son of God, that he descended from heaven to save us, and that he died to absolve our sins.

The Battle of Milvian Bridge was fought in 312 CE, between rival forces in the Roman Empire. According to primary sources, Constantine had a dream in which God visited him and instructed him to fly the Christian cross as his standard. Constantine, who won the battle, felt like this was divine intervention and enthusiastically embraced Christianity throughout the remainder of his life.