The correct options are
A skill
B birth
The Rig Vedic society had divided itself into the four varnas - Brahmins, Kshatriyas, Vaishyas and Shudras.
During the Early Vedic age, the segregation was mostly according to their skills and not by birth.
During the Later Vedic age, the segregation was strictly based on birth. This led to the formation of a rigid caste system.