Optional Subject should be chosen on the basis of one’s interest in that subject, scoring pattern of that subject in previous few years (see here), availability of study material and expert (Click here) guidance.
An optional should be generally the subject studied during graduation/Post graduation. (There is no hard and fast rule in this regard, but my advice to the aspirants is that choosing “parent subject” as your optional proves beneficial, until and unless there are some very genuine points against that subject).
How to choose optional subject for IAS main examination DIASIndia