Hello Piyush,

Actually, you should have secure at least 75% marks in the 12th class board exam, or be in top 20 percentile in the 12th class exam conducted by the respective Boards. Then only you can eligible to get NIT colleges and to appear in the jee advanced exam.

Improvement for one or more subject – If the candidate appears for improvement in one or more (but not all) subjects then the 12th percentile marks of their original passing year will be considered.

Improvement for all subjects – If the candidate appears for improvement in all subjects, the percentile marks of the present session will be taken into consideration.

For more information, you can go through the link given below

https://engineering.careers360.com/articles/jee-main-eligibility-criteria

Thanks for query

All the best.