Sir I am 12Th Science and Left BSc. I want to complete BA in one sitting
BA is a three year Under graduate degree program. So you can't complete it in just a year. You have to enroll yourself either in Distance Education Universities or the regular universities for BA course and have to qualify all the semester in order to get a BA degree.
I hope you got my point. All the very best for your future endeavors!
Sorry, but your question is not clear. If you want to joine BA course instead of BSC which you left, you can join BA and complete graduation under any specific subject stream as per your interest. There's no problem in that. Just look out for the ekhi criteria for the course in the college you are looking into.
Hope this helps.
Good luck!