Hi,

There is no as such compulsion in IISER Aptitude Test to attempt all the sections. If you wish you can do any one section among Mathematics/Biology. The cumulative score in the exam is considered. If you wish you can attempt all the section to increase your marks, if not, you can either skip Biology or Mathematics as per your preference.

IISER Aptitude Test Syllabus, Exam Pattern, Dates: https://engineering.careers360.com/exams/iiser-aptitude-test