Many people have asked this question and i wrote this reply to someone on this very forum. I think it may help others as well.
Choosing optional is the most difficult task in CSE exam. I also faced similar problem and I couldnt get any good gyan from internet. All I got was fatte and I went with the wind of that time PubAd. So, after 3 years into this area, I think the following method would be best suited objective evaluation and help you in choosing your optional.
This is an objective way to reach an optional best suited for a particular person in general. Of course there can be exceptions.
1) What is your field of interest- Always pick what excites you cause IAS exam is a marathon and not a sprint which can be done away with pure muscle power. If you like what you read and enjoy it, then you can top it. Gaurav Agrawal, Rank 1, 2013, took the most unconventional optional which was Economics cause he liked it and he was good at it. The sheep mentality in UPSC doesnt work anymore, where everyone used to take PubAd and more than 60% of the selected students had PubAd as their optional. If you like your subject you can dig deeper and the optional wont be a burden to you. So pick up XII NCERT of any subject that you feel you like, and see if you are good at it. (Almost all subject has NCERT on it even Business Administration). And remember you should be able to learn even the most boring part of your chosen optional. Cause, every optional has a boring part. Even if you love History or Maths, there are many boring parts in it and thats what makes the mastery of optional very difficult. Give 5 weightage to it.
2) Availability of subject material - This is the second most important thing. If you cant acess great material, then the battle is already lost. You are done. Even with best brains and aptitude in the given optional you cant make it. There is dearth of good optional material in PubAd. No coaching has yet come up with comprehensive material on pubad adn claim that 90% of teh questions will be from their material or notes. Even the standard books are not enough. However in sociology, the material avaialable is very good as it is taught at university level throughout the world and you can get tonnes of material. Same is with Geography or maths etc. Give it 4 weightage
3) What you have already studied lessens your burden. Choosing an optional from your graduation year can be a great boost. As you are already familiar with the subject. All you have to do is expertise in that subject. Taking a new optional is not the best idea as you have to start from scratch and you will have to do 5 year course in one year. And also there is a very good probability that what you chose in graduation was mainly because you were good at it and so choosing that optional will help boost your performance, take less time, bore you less and will be less of a burden. Gaurav Agarawal chose Economics cause he had studied it extensively in his MBA program and also cause he was interested in it. This made it easier for him to concentrate on other new areas GS1 to GS4 and essay. Tina Dabi Rank 1, 2015 had studied Political science in her 10+2 and in graduation as well. ie 5 years of continuous exposure made her good in her optional and required lesser preperation and lesser time to develop writing skills needed for her optional sparing her more time for GS and Essay. Give it 4 weightage.
4) Dyanamic/Static nature of the subject. Very important in current scenario. The pattern of current exam is such that in dynamics subject such as PubAd and IR(International relations), the landscape is changing quickly. As you might have heard, PubAd is still not a fully developed subject and its roots are very new. And so a lot of new changes keep taking place. IR also changes very rapidly due to so many conferences and meetings. Thats why people are not able to get good marks in GS2 due to dynamic nature of subject IR. But IR still has the advantage of being discussed widely and so material is available. PubAd is an esoteric field, only understood by a few professors in IIPA or the working IAS at top rung of their job who have undergone good amount of training and very few reports. ARC 2 reports are out dated now. Almost 10 years have gone by. Static subjects such as Psycology, History, Maths etc are doing good in such new scenario. So, I will give this aspect 4 weightage
5) Availability of proper Guidance There is a need for proper guidance on your subject. It can be through a blog of topper or through coaching institute or through some topper you know and is ready to guide you. There is tonnes of material to be studied for optional and if you are not sure what to study and what to leave, you are going to decrease your cost-benefit ratio. In fact what you dont need to study is more important than what you need to study. Cause many times we study useless books and find our time is gone. Here guidance is very important. But still, if you have great enthusiasm for your optional and can analyse your weakness and concentrate on what to study without being affected by market forces(i.e. millions of books and magzines in market), then guidance may not be that important as happened with Gaurav Agarwal with his Economics optional. There was not much good optional guidance available but he still made it after studing million new papers from EPW. In PubAd also, good guidance is not available. So, I will give Guidance3 weightage.
6)Optional doing well This can be one way to choose. Sometimes UPSC does favour a few optionals. Antropology did great during the 1980s. During 1995-2000 it was science especially Maths. Then from 2002-2012, PubAd had a dream run. 2012-2014 Sociology, Philosophy and literature were doing good. Literature is still doing great. Law, Maths and Medical science are other optional that are doing great. Someone pointed out that for past 3 years, one question in Pali has been repeated every year. And students give special focus on that question and get good marks out of 25 marks. Also Literature according to many have very small syllabus and can be completed in 15days to 1 month leaving a lot of time for other subjects. Anthropology seems to make a comeback these years. That was one of the main reason, UPSC tried to delete the Literature optional in 2013 but public pressure forced them to make it stay. But, still I dont think that is a good way to choose optional. So if your choice of optional is within currently good optionals, you can give it a try. But you never know, UPSC can give stepmotherly treatment to any optional any year. This has happened to many and can happen to you. So, give UPSC current favourite optional 2 weightage.
After doing this objective exercise and reading basic NCERTs of optionals you are trying to chose for a week, you can arrive at an optional best suited for you. Hope it will help you all new aspiring students.
Good Luck to all.