The optional paper in the UPSC mains exam includes a range of subjects tailored to individual interests and expertise. Popular subjects consist of Geography, Politics, History, and Sociology. Other options include Public Administration, Anthropology, and Literature from various languages. Candidates must choose a subject they are comfortable with, as this significantly influences their overall performance. The choice of the optional paper can reflect a candidate's academic background, interests, and career aspirations in the civil services, playing a crucial role in their success.