If you want to take only the Biology exam from the National Institute of Open Schooling (NIOS) after completing your 12th grade with Mathematics, you will need to follow the guidelines set by NIOS for the subject.

NIOS offers flexibility to students by allowing them to appear for additional subjects after completing their board exams in other streams. Since Biology is a science subject that includes both theory and practical components, you will be required to take both the written theory exam and the practical exam, even if you're only adding Biology as an additional subject.

The practical exam is mandatory for all science subjects, including Biology, as it assesses your hands-on understanding of biological concepts. You will need to prepare for the practical exam by studying the prescribed syllabus, which includes experiments, lab work, and practical skills outlined by NIOS.

Ensure that you enroll in a study center that offers practical sessions, as these are essential for preparing for the exam. Once you clear both the theory and practical components, NIOS will provide you with a certificate for the Biology subject. Make sure to check NIOS's official website for updates on exam schedules and practical exam guidelines.