Commonly chosen subjects as optional in the UPSC Mains exam include Political Science, Public Administration, Sociology, Anthropology, and Geography. These subjects tend to appeal to aspirants due to their relevance to the civil services and the overlap with General Studies content. Additionally, courses like History and Literature are popular among candidates who have a background or interest in these fields. Choosing the right optional subject plays a vital role in maximizing scoring potential and aligning one's strengths and interests with the examination requirements.