Hi

Yes, you can definitely take biology as an additional subject in your 11th and 12th . Additional subject is nothing but your 6th subject. One is free to choose any additional subject of his or her choice provide his school offers that subject.

For example,

I had chosen Hindustani music as an additional subject as I already had both maths and biology as main subjects. While one of my friend didn't take any optional subject while another one chose biology as an additional subject, maths was his main subject. So, it's all your choice.

Also,

Students with Biology as an additional subject in class 12 board examination are provisionally eligible for Neet examination so in case after your class 12th you decide to appear for Neet examination in order to have a career in the field of medical by pursuing MBBS /BDS etc then you will be eligible for that as well.

Thank you