if have taken pcb in class 11 abd now during hsc board exam registration form can i opt to take pcmb

If you mean Maharashtra HSC, and you took PCB in Class 11 but now want to add Mathematics and make it PCMB for Class 12, it may be possible only if your Junior College allows the subject change and you meet the Board's subject-combination requirements.

The Maharashtra Board officially lists Mathematics & Statistics and Physics, Chemistry, Biology among its HSC subjects.

However, simply selecting Mathematics in the HSC board exam form does not necessarily mean you can add it at this stage. Your Junior College has to approve/update your subjects in its HSC records before the board application is finalized. The Board's HSC exam applications are submitted through the college/institute system.