My dear student , in class 12 if u choose science..then there is 2 options like 1.pure science and another one is 2.bio science .In pure science math, physics, chemistry is must and here biology canbe ur optional or 4th sub ..but in bio science ...here biology is must..
IF U WANT TO GO FOR NEET or any others medical line then u have to choose biology ..but IF U GO for engineering..then math is compulsory..
HOPE IT HELPS.
Thank u...

Hello aspirant

You can skip one of the two subjects either maths or biology while filling your form for STD xi. You have this choice depending upon which field you are going to choose after xii Science. If you want to pursue any medical course you will have to select biology compulsorily along with physics and chemistry. If you want to go for Engineering course either BE or B tech you must take maths compulsorily. Some students select both the subjects ie they take PCMB combination. Some courses such as biotechnology the candidate must have studied  both biology and mathematics in STD xii.

They are compulsory subjects. You have to choose either of the one.