Overview

addOption and removeOption does not work with Multiple Choice variables. This works with Select Box and its Question Choice. 

Additional Information

This is OOB behavior as of now. 

***Enhancement Request***

If you feel this has great business impact on your end, you can raise enhancement request. 

Please check following KB to know how to raise enhancement request.
https://hi.service-now.com/kb_view.do?sysparm_article=KB0547257