Selecting the right optional for IAS is crucial for scoring well in the Mains exam since the two optional papers carry 250 marks each and account for 28.5% of the Mains written marks. But the problem is that since 2014, UPSC has made it compulsory to select the optional subject while filling the Prelims form itself and you cannot change it while filling the Mains form.
So if you’re unsure about selecting the optional subject, it is advisable to wait while there is still time to apply for the Prelims and make up your mind before finalizing the optional subject.
You can go through this post on selecting optional subject for IAS exam to decide which subject is best for you. As stated in the post, your interest in the subject is more important than scoring criteria. Why? If you find the subject interesting to read and write about, the marks will follow. But if you find the subject boring and a chore to read, you will not add anything new or unique to the answers apart from the routine matter resulting in average marks.
Availability of study materials is also an important requirement before selecting the optional paper. You might want to choose Public Administration as it is the most popular optional subject among IAS aspirants. but you need to keep in mind that Pub Ad is also dynamic and you need to supplement Paper 2 notes and books with regular newspaper and journal reading, besides the ability to apply concepts in governance. Also, simple illustrations by way of block diagrams or flow charts can result in extra marks.
If you’re still not sure which optional subject is right for you, you should go through some basic books for the optional subject you are considering. You need not buy the books initially, rather you can borrow from your friend or from the local library or just search online in Google book search to get some idea about the subject.