Dear aspirant,

As you have mentioned that you have done your 12th with pcm from HSC Maharashtra board in the year 2018 and now you wanted to take a seperate exam of biology so let me tell you that's not possible because your 12th marksheet has been released and you have already graduated from your school.

If you are talking about some competitive examination which is related to biology then you should have taken biology with physics chemistry mathematics as your subjects only then you would have been in eligible to crack these exams.