Optional subject selection is crucial with 500 marks at stake. Popular choices include: Anthropology (suitable for all disciplines, helps with GS-I); Geography (high success rate, overlaps with Prelims); History (static nature, good material availability); Mathematics (objective, high-scoring, static syllabus); Philosophy (highest success rate, less syllabus, helps with GS-IV Ethics). Choose based on interest, material availability, faculty access, and ability to study for 3-4 years. Avoid following trends or guarantees.