Many people who’ve graduated or done Masters in some subject opt for it. This is really helpful as you’re already familiar with the subject and will need to study just one subject from scratch.

Yes, but of course you can select those subjects also which you have not studied in graduation for IAS exams. You can select any subject/combination of subjects from the list of subjects available for the purpose as Optional Subjects for UPSC prelims & mains exam

However, the Following Combinations are NOT allowed by the UPSC

Political Science & International relations and Pubilc Administration
Commerce & Accountancy and Management
Anthology and Sociology
Mathematics and Statistic
Agriculture and Animal Husbandry & Veterinary Science
Management and Public Administration
Any two branches of engineering
Animal Husbandry & Veterimary Science and Mediacal Science
Combination if two Literatures