Hi Prashant,

There are many subjects that you can take to prepare for IAS. It doesn't matter which specific subject to select. The subject that include in syllabus are mostly covered in school in the name of general studies.The subject in civil services are designed to judge you on your understanding of things around you. I would like to suggest you to take such subject of which you have detail background of that subject or you have previously learned that subject so it will be additional benefit to you. You can take Polity, Economics, History, Geography, Environmental ecology, Agriculture, Medical science, etc any of these. Out of these you can select one to which you are well known and comfortable to you, also at the same time you must find the interest in that subject because you have to study that subject in detail whole year. So choose your subject accordingly.

All the best!!