Dear Rachit ,

mostly school allows to take one optional subject with PCM (majorly Computer science or Hindi or Physical education ). So, you can not take the sixth subject as optional .

But if you want to switch your optional subject to political science , you can . For that you have ask your school' permission . And then you have to write one letter or so  to CBSE (please ask your school for that). Generally It is very long process . And it might happen you have to give exams for both the subject . So be careful.

Thank you.

-Tejas ladhani