Yes, all four sections physics, chemistry, mathematics and biology are compulsory for IAT but their is no sectional cut-off.
Most of the students who appear in the IISER Aptitude Test will have either PCM or PCB in Class 12 and only few will have PCMB as subject.
So, do really very well with pcb so that you can score for atleast 12 out of 15 questions and as it's difficult to conceptualize 11th & 12th mathematics, start learning formula cheat sheet available for jee and something. By scoring for 12/15 questions in other sections it's sufficient to score for 5 question out of 15 in mathematics.

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