Candidates can choose from a variety of subjects for the Mains examination of the UPSC, typically starting with Indian languages, English, and expanding to various optional subjects. These optional subjects include categories like Anthropology, Public Administration, Geography, and many more across categories such as Humanities, Social Sciences, and Engineering. Aspirants must carefully select subjects based on their strengths, interests, and background to optimize their performance in the examination, as each subject demands in-depth knowledge and skills in writing.