if I failed in 1 main subject in class 12 board will optional subject replaces it's marks I have 6 subjects in 12th .
Dear aspirant,
No, this is not the case. You can be failed in any of the optional subject but you have to clear your main subjects. For example if you have music and physical education as your two optional subject then you can be fail in anyone of them and pass in another and you're overall passed.
However you that will not be the case with main subjects.