First, you must be interested in the discipline. Evaluate your aptitude to learn the subject; the choice should not merely see you graze through to the next level. Don’t let the length of the syllabus influence your decision. History and geography are known for their notoriously lengthy content whereas public administration and anthropology have the least topics to learn. The secret does not lie in selecting subjects which are short or lengthy but on how comfortable you are in grasping them.
Here’s a litmus test. Pick up any basic book of the subject and read the first chapter. Are you keen to read the next chapter? How much do you recall of what you read? Are you ready to make notes for the next three hours? Assess these factors and you will know if this is your second optional.
The next vital step is the availability of resources. Find out how easily study material, guidance and coaching are available. If you have studied the discipline then you will have a fair idea about the syllabus and also the source for books. But if it’s a completely new subject then it’s sensible to speak to a senior who will be able to guide you.
Source: (Optional Subjects For IAS Exam )http://www.iupsc.com/optional-subjects-for-ias-upsc-exam/