The answer to this question can only be found on the basis of your interest. If a subject interests you and you want to learn more about it, you are bound to score good marks. Also, if you have completed your graduation in a particular subject which is also an optional according to UPSC, going for that subject will be fruitful too. But, it all comes down to your desire to read and understand the subject. Just because an optional has been doing well in the papers doesn't mean that you will be able to score that well. Thus, the most important criteria to choose your optional is your interest/desire to learn that subject!