In which one of the following Vedas does the term 'Gotra' appear for the first time as representing a clan?

The term 'Gotra' literally means "cow-pen" or "enclosure for cattle." In the Rigveda, the earliest of the Vedas, the word appears frequently but primarily refers to a physical space where cattle were kept or to a herd of cows. It did not yet signify a kinship group.

The transition of the term to represent a clan or a patrilineal descent group occurred during the later Vedic period. It is in the Atharvaveda that 'Gotra' first appears with the meaning of a group of people descended from a common ancestor. This period marked the institutionalization of the gotra system, which established rules for exogamy (marrying outside one's clan) and became a core feature of the Brahmanical social structure.