Sign of principal in exam form

yes , if your principal is not availble make it sign by other icai member and send a copy of articles completion confirmation letter with it ...

after completing articleship you can sign your form from any CA 

but why in the form it is mention only the principal. 

Even I had the same query, but it is only applicable for students who are presently in article training. Many students will take the sign from any member of the institute after completion of their articleship. So, you need not worry. You can take sign from any member of icai.