Take full and differential backup on secondary replica - SQL Server 2014
I need to setup full-backup and diff backup. Can I do it on Server C?


You can do a copy only full, but unless C is the primary you cannot do a differential database backup.


  Running below query on server C but it throwing 1 which means I can't take backup on that server.


A return value of 1 means it is "preferred" but doesn't necessarily mean you can run any type of backup on it.