I prefer to join aerospace engineering,but if I don't get the seat , what should be my second option (courses)
see apart from aerospace engineering, you could look at aeronautical engineering offered I think by Manipal University, SRM to name some. You could apply to IISt - they have avionics too. However admissions to IIST is through JEE Advanced.
The next option in case these dont pan out should be electronics because basically all these fields generally have a close connect to electronics so you could move on further to do a postgraduation in aerospace etc I guess.
All the best!