As per latest syllabus of CBSE the compulsory subjects for a Bio student are as given below-

1)PHYSICS
2) CHEMISTRY
3)BIOLOGY
4)ENGLISH
5)OPTIONAL- Any subject you like!

But for a science background especially that of PCB one must take an optional subject which is easy for the person to go with.
Let me tell you all the optional subjects too.
1- Sanskrit- bit easy (only if you are well versed with the basic concepts)
2- psychology
3-Economics
4-Hindi
5-Physical Education ( you have to do crunches and lunges)
6-Fine Arts
7-Maths ( for commerce and bio students)
8-Information Practices and C++

Choose a wise option :)