The selection of an optional subject dramatically impacts a candidate's overall score in UPSC Mains due to its specific syllabus, scoring patterns, and personal affinity. Candidates should choose a subject based on their strengths, interests, and availability of resources like textbooks and guidance materials. An optional subject with a smaller syllabus that aligns well with general studies can optimize preparation time and enhance scores. Furthermore, the ability to write well in the chosen subject can improve overall writing proficiency, beneficial for both optional papers and general studies.