In the context of tribal assemblies, the option "Samiti" is the correct answer. Samiti refers to a gathering or assembly of people in Indian society, especially in ancient times. It was a democratic institution where decisions were made collectively, including the election of the tribal chief. Sabha, Gana, and Vidata are also types of assemblies but not specifically involved in the election of the tribal chief. Sabha typically refers to an assembly or council of learned individuals, while Gana refers to an assembly of common people. Vidata, on the other hand, refers to a tribal council or assembly responsible for decision-making in administrative matters. Therefore, option 1, Samiti, is directly related to the election of the tribal chief.