Christians are referred to as 'people of the book' by Muslims.
Answer: B. Christians
Explanation
It is a term used to call Christians, Jews and Sabians by the Muslim people.
This reference is used in many parts of the Quran.
This term shows the belief of these groups in one God.
They referred in many parts of the scriptures especially in the Torah, the Psalms and the Gospel in the context of the Quran.
This is showing a form of respect to the virtues of these people.
In history, this term is used by different religions further.