You should fill the option of Central Board of Secondary Education in the form. It is very easy to fill the form so don't take any tension. Kindly check all the eligibility criteria before filling the form. Best of luck for your exams and I  hope you qualify it. 

Hi Aviral 

you have a cbse option for the board selection you can select cbse board.

here is a link for your help:https://www.careers360.com/exams/bitsat

All the Best