Which document standardized Christian beliefs?

the Old Testament

the New Testament

the Dead Sea Scrolls

the Nicene Creed

I believe the answer is: Nicene creed

Nicene creed filled with statements from the orthodox faith of the early Christian church which oppose several heresies. This statement of belief is widely used among Christian Liturgy and also used as profession of faith for those who want to obtain important position in the catholic church.

I chose the New Testament. Though Christians refer to the Old Testament, it in the New Testament that Christian derives their teachings. The faith is now open to gentiles and there now more teachings that revolve on doing good deeds and honoring God.