Step 1: Concept Identification: The query requires the identification of an "acquired" trait from given options used for tribal classification. An acquired trait is developed during an individual's lifetime, contrasting with innate or inherited traits.
Step 2: Analysis of Options:
(A) language: Acquired culturally through community and family, not biologically inherited. This is a primary acquired cultural marker.
(B) livelihood: Learned occupational practices, often influenced by environment. While acquired, language is a more fundamental cultural classifier.
(C) racial features: Genetically determined physical characteristics (e.g., skin color), which are innate.
(D) habitat: The geographical environment of a tribe, an external factor, not a trait of the people.
Classification of Indian tribes utilizes both inherent (racial) and acquired (language, livelihood) traits. However, language stands out as the clearest acquired cultural trait used for classification (e.g., into linguistic families).
Step 3: Conclusion: Language is an acquired trait employed in tribal population classification.