Hello student NIOS is a recognized board so yes you're eligible as eligibility criteria says - Candidates must passed or appearing class 12 with physics and mathematics for Airforce and Navy and class 12 in any stream with for Army.

Candidates must lie between 16.5 to 19.5 years.

For more updates refer to the link mentioned below - https://www.google.com/amp/s/competition.careers360.com/articles/nda-eligibility-criteria/amp

Feel free to comment if you have any doubt

Good luck