Subject Scalability: It should be of the highest priority to consider how (comparatively) easy it is to score in a given optional subject. It would be wrong to consider that all the optional subjects in the UPSC are equally scoring.
Students’ Own Abilities: One cannot be equally good in all the subjects and it is important to recognize what is the most suitable option for you and act accordingly.
One’s Own Interest: The subjects that you really love to study will make the journey at UPSC very easy. You will not easily get tired of studying it, the moments of monotony are quite less likely to occur. The interest that you have in a subject is one of the criteria that should be given primacy.
Commonality (Overlapping with GS): If an optional subject is also helpful in General Studies or an Essay or the Interview, then it automatically increases the preference for the particular subject.
Length of Syllabus: Optional subject must be chosen keeping in mind the time in which the syllabus can be covered. If you are able to save one or two months in a subject then it means a lot from the CSE perspective.
Sources of Information/Study: Other important aspects for choosing the right optional subject include having enough study materials and proper guidance from seniors and teachers.
The dynamism of the Subject: Lesser the less dynamism, the better the subject. In some subjects, questions are the same but answers change every year.
Background: However, not as significant as others, but if you have a serious background in a subject that also satisfies other important criteria of choosing optional, then the background should be given a good weightage in the selection of the subject.
Time Management: Apart from the comprehension ability, a candidate should decide how much time he/she would be able to spend on studying an Optional Subject.