Firstly, it is not mandatory that your optional subject should be from your stream. It may or may not be the subject of your background. If you are too good at your own subject and can easily solve the paper on the same subject asked by UPSC then you should definitely have it as optional subject. But if you are not able to solve then you should switch on to some other subject. For detail answer read How to Prepare section of our website or visit ARIHANT ACADEMY.