The stream SB is for those Students enrolled in 1st year in basic science will go under this stream. So if you are in the first year of your BSc Physics then you are eligible for the KVPY. If you have been in your 2nd or 3rd year then you aren't eligible for the examination.

To know more about KVPY visit the link below:

https://school.careers360.com/exams/kvpy

I hope this answer helps. All the very best for your future endeavors!