Firstly if you have strong background in any subjects such as if you have done graduation in mathematics and you are very good student of mathematics then obviously your first should be your graduation subject but in case you are not specifically interested or expert in any subjects then most of the students choose History or  geography and public administration because this 3 are considered to be most scoring subjects in optional civil services exams therefore it is better if you don't have any specific choice to choose among these three subjects.