Correct Answer: Purusha-Sukta of Rig Veda
The earliest application to the formal division into four social classes (without using the term varna) appears in the late Rigvedic Purusha Sukta (RV 10.90.11–12), which has the Brahman, Rajanya (instead of Kshatriya), Vaishya and Shudra classes emerging from the mouth, arms, thighs, and feet at the sacrifice of the primordial Purusha, respectively.