i want to do BA and which subject and college should i opt so that i can get more time to study for competitve exams like NDA and other government exams for officer level

You can choose any subject in B.A. every subject is useful for you for govt exams and NDA. If you want to have career in Defence then you can choose B.A. Defence and Strategic studies it will be very useful for you in future. If you want to have another subject then choose history because it is very helpful in other exams. But other subject as English, geography, public admin, political science etc many courses are there and all are helpful for you.