Hey!
Yes biology is the mandatory section to attempt iiser aptitude exam. If you haven't study biology yet, then you must start preparing. Exam consists of basic questions from 10+1 and 10+2 biology You can simply prepare from ncert books. For more information you can go through this link
https://www.google.com/amp/s/engineering.careers360.com/articles/iiser-admission/amp
Good Luck
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