Hi

A student can definitely appear for class 12th board exam again.

But there is no provision of giving one specific subject's exam. You have to give exam of all subjects of class 12th in order to be eligible for the same.

National institute of open schooling (NIOS) is a government aided scheme to help students give class 12th exam, in case of change in board, increase percentage.

No, there is no provision about gap in exams, any one can give exam, considering the age >17 years

Best of luck