When selecting optional subjects for the civil services exam, candidates should consider factors like personal interest, strong foundational knowledge, and syllabus overlap with other subjects. Researching the scoring trends of various subjects and understanding their relevance to current topics is also advisable. Consulting seniors or mentors who have navigated the selection process can provide valuable insights. Ultimately, choosing a subject aligned with one's strengths while being manageable within the preparation timeline will enhance focus and scoring potential.