In the present set-up, Optional Subject selection is not that difficult. But, for candidates like you, it is little tricky as there is no optional subject available that directly match with your academics.
However, Optional Subject selection is not that challenging once you commence serious preparation.
The syllabus for General Studies at both Prelims as well as Main Examination involve many themes from diverse disciplines that gives you a feel about subject that may be picked as optional subject.
The following article would help you in getting some insights about Optional Subject selection: