Hello Deepak!

If you only appear Maths in NIOS, you will become eligible for appearing JEE Mains but problems will arise when you will sit for JOSAA or CSAB counseling because you can cannot provide two marksheets at once.

As per the NIOS website, it does not provide pass certificates/marksheets for only one subject. A student needs to have 5 subjects to get a certificate. So, it is better if you appear for on demand exam of NIOS and clear two papers and also apply for TOC ( Transfer Of Credit) for getting the marks of other 3 subjects(i.e Physics, Chemistry and English/or any other subject).

In this way, you can apply for IITs with a single marksheet. If you sit in counseling, and provide marksheets of two different boards it will be problematic.

Hope you are clear!