Hi Sachin

Sorry to say but you can not give 12th board examination again that too after getting enrolled in an university for BCA degree.

It will be considered if you give examination from a state board while being enrolled in a college. If you really want to get int defence field then what you can do is ,leave the course you are doing and focus on BSEB result and get readmission in college with fresh transcripts.

HOPE IT HELPS

ALL THE BEST