Hello aspirant.

No, Biology is not compulsory in iiser, in case any subject is not compulsury for iiser test.

You can take any of the subjects.Especially since the common subjects- Physics and Chemistry - are not crazily tough, hence a lot of people will score well here. Also, if you are a Math / Bio background student, you will obviously be expected to score well in your specific subject.

Hence, for a math background student, it is somewhat essential to ensure that you take atleast some points from the bio section to supplement your total score. This will give you a good advantage over a large number of people.

Hope this will help you.