The doctrine of moksha or liberation receives its first sustained philosophical treatment in the Upanishads, where knowledge of Brahman and Atman is presented as the way beyond the cycle of birth and death. The Upanishads fits that evidence because it matches the specific chronology, agency and function contained in the question. A comparison with the alternatives is useful: The Rigveda, The Brahmanas and The Aranyakas are genuine historical terms or names, but each belongs to a different office, patron, text, region or sequence. The compact memory link is therefore The Upanishads together with the factual context above, not the isolated option letter.