Hello  Anagha, 

It is not compulsory to ‘attempt’ biology but they will consider its marks in the total i.e. your marks will be calculated out of 180.

Though it's not mandatory to attempt the biology section in the IISER Aptitude Test, but for a good score you can't omit that section. Please remember that you won't be disqualified even if you don't attempt any question from the biology section. But the thing is that your score will be hampered in total. 

Now let's have a look on the question paper pattern of IISER Aptitude Test :

• There are 60 questions in total  in the test.
• 15-Physics
• 15- Chemistry
• 15-Mathematics
• 15-Biology.
• Each question fetches 3 marks.
• Your total marks are therefore 180.
• No Sectional Cut Offs are there. 

Now, let's see the syllabus for Biology :

• Diversity in the living world
• Structural organization in animals and plants
• Cell structure and function
• Plant physiology
• Human physiology
• Reproduction
• Genetics and evolution
• Biology and human welfare
• Biotechnology and its application
• Ecology and environment

So, as you see the syllabus, it covers the 10+2 NCERT Level. It would be useful to study 10+2 NCERT Book for each of the 4 subjects. It's more than enough to have a pretty good score.

The most important factor is that each and every year, the competition among the aspirants are increasing. Therefore, scoring high should must be your top priority to crack IISER Aptitude Test. The common subjects, i.e. Physics and Chemistry wouldn't be so tough. Each and every aspirant will score good in Physics and Chemistry. But the thing is that if you are from a Maths/Bio background, you must score good in your field. You need to score above 30–35/60 to get admission in any IISER. Don't attempt the question about which you aren't sure because there is negative marking also. So be careful while answering the questions.

Hope this helps!!

Good Luck :)