Hi Suresh,

Yes, you can give exams through NIOS even if you fail in the improvement exam of class twelfth.

But I would suggest you to rather reapply in the succeeding year from your school itself than opting for NIOS for improving your overall percentile. NIOS has a bad image and might provide hurdles in future even though it is a recognized board some less aware people often ask questions and may refuse to give admission in some colleges which should not be the case because, in the end, it is a recognized board of education.

If you scored 75%, and you want to improve it, you can re-apply privately(in CBSE ) through your school, and opt either for selected subjects you want to improve your marks in or for all the subjects altogether. CBSE will issue a statement of marks after the improvement of performance.

Refer the below link (Especially question no 9 from FAQ’s) for more information:

http://www.cbse.nic.in/examanxiety.htm

For any other query or more details, feel free to ask.