The correct option is B Birth
Varna system was based on Birth, during the early vedic-period. The varna system is discussed in Hindu texts, and understood as idealised human callings. Traditional Vedic organisations and scholars say that, even according to the Bhagavad Gita the Varna is birth based only. Bhagavad Gita is considered as the essence of Vedas and Upanishads so why would it say anything which contradicts Vedas and Upanishads.