The correct option is B A is true and R is NOT the correct explanation of A.
The Purusha Sukta in the 10th Mandala of Rig Veda mentions about the hierarchy of four classes: Brahmins, Kshatriyas, Vaishyas, and Shudras. This codification about the four classes is one of the evidence for the genesis of varna system during the Rig Vedic Period.