Hello Divyank,
According to the JAC Delhi Eligiiblity Criteria for B.E/B.Tech course in NSIT, the candidate must score at least 60% marks in Physics, Chemistry and Mathematics taken together.
You are lacking with few marks only. Since you are appearing for the improvement exam, you can improve your aggreagte marks. So, then you will be eligible for admission to NSIT Delhi.
Also, make sure to score a good rank in JEE Main exam also. Merit rank of JEE Main paper 1 will be considered for admission to B.E/B.Tech courses.
Good Luck!