BACKUP DATABASE supports only copy-only full backups of databases, files, or filegroups when it's executed on secondary replicas. Copy-only backups don't impact the log chain or clear the differential bitmap.
Incorrect Answers:
A: Differential backups are not supported on secondary replicas. The software displays this error because the secondary replicas support copy-only database backups.
Reference:
https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/active-secondaries-backup-on-secondary-replicas-always-on-availability-groups