See the total marks for the WBCHSE is calculated on the basis of best of five. So if you fail in one subject which is your optional one, then still you will be consider to be passed the examination. The passing marks is 30% of the total marks in each subject. If you have practical subjects, then you have to secure 30% both in theory & Practical in order to pass the subject.

For more information regarding WBCHSE you can consider the link below:

https://school.careers360.com/exams/west-bengal-12th

I hope this helps. Wish you a great eve!