So can i change just the optional subject . I am in mathe science stream . As per these statements:

(I)Private candidates shall not be allowed to offer for their examination, a subject (even if the subject is recognised for the examination) which is not being taught in an affiliated School.

(II) Private candidates shall not be allowed to offer such subjects for the examination which involve practical work except in the case of candidates who had failed earlier and who had put in a regular course of study at an institution affiliated to the Board in the previous academic year.

I can not choose for any subject which contain any practical work. And hindi does not contain any practical at all.
And hindi is also taught in my old school as an optional subject.