Every student has to select one optional subject in UPSC CSE which carries 500 marks. A student can choose anyone from the given list which can be the same either same subject in which he has graduated or others. So there is no compulsion to chose the same subject in which you have graduated. You can choose another subject also.
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List of optional subjects
(ii) Animal Husbandry and Veterinary Science
(vi) Civil Engineering
(vii) Commerce and Accountancy
(ix) Electrical Engineering
(xvi) Mechanical Engineering
(xvii) Medical Science
(xx) Political Science and International Relations
(xxii) Public Administration
(xxvi) The literature of any one of the following languages:
Assamese, Bengali, Bodo, Dogri, Gujarati, Hindi, Kannada, Kashmiri, Konkani, Maithili, Malayalam, Manipuri, Marathi, Nepali, Odia, Punjabi, Sanskrit, Santhali, Sindhi, Tamil, Telugu, Urdu and English.
An aspirant has to chose one optional subject. It can be either your graduation subject or other.
No, you can not change the optional subject after filling DAF.
You can choose another optional subject in the next attempt.
Yes, you can choose any subject as your optional subject irrespective of your subject in graduation.