The correct answer would be option D, Belief system in a God or Gods with certain rituals and/or literature.
Explanation:
There are many religions in the whole world like Christianity, Islam, Buddhism, Hinduism, etc. All religions have different beliefs which bind the people of that religion with one another.
So the most important things the members of religious groups share are the belief systems in a God or Gods, and certain rituals and/or literature.
With the help of an example, the above statement will be clear.
I am a Muslim. We, all Muslims believe that there is no God but ALLAH, who is the creator of everything. This is our common shared belief on our God. We celebrate our Eid days every year after fasting for the whole month. This is our shared rituals. We believe on the Holy Book of Quran, which is sent to us through our Prophet Muhammad PBUP by our ALLAH. This is the literature we all believe.
So in this way members of a religious group share beliefs system, rituals and literature.
Learn more about Belief systems at:
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