Can a candidate choose an optional subject, which he has not studied at graduate/post graduate level?

Yes, a candidate can choose an optional subject which he/she did not have as there graduation/ post-graduation subject. However, there are few things that you should keep in mind while figuring your optional. Firstly, do not get carried away by the advice of anybody else while deciding your optional because different coaching institutes and different successful candidates have their own advice to give. These advices which are given in good spirit are based on individual strengths and preferences which may not be wrong but it might not be suitable for you. Secondly, you should notice your inclination and orientation to a particular subject which you like and that should be your preference for your optional as you tend you spend more time on the subjects you love. Thirdly, try seeing that the subject you decide has a general studies overlap. Lots of optionals have a general studies overlap and to know this you should have a thorough look at the syllabus. By taking the subject as optional which has a general study overlap, it will also help you to build the foundations of both the subjects simultaneously. Fourthly, you should try and understand the current trends that is going on. There are trends of different subjects which emerges and vanishes. Of all the above factors mentioned for choosing your optional, liking for any subject is the foremost important criteria for choosing your optional subject.