Candidates select their optional subject in the UPSC Mains Examination based on multiple factors, including personal interest, academic background, and scoring potential. It is advisable to choose a subject that aligns with one's strengths and knowledge base, allowing for deeper understanding and engagement. Candidates should also consider the overlap of the optional subject with the General Studies syllabus, as this can enhance their preparation efficiency. Additionally, analyzing previous years’ trends and cut-offs helps in making an informed decision tailored to individual capabilities and preferences.