Your network contains an Active Directory forest named adatum.local. The forest contains 500 users and uses adatum.com as a UPN suffix.

You deploy a Microsoft 365 tenant.

You implement directory synchronization and sync only 50 support users.

You discover that five of the synchronized users have usernames that use a UPN suffix of onmicrosoft.com.

You need to ensure that all synchronized identities retain the UPN set in their on-premises user account.

What should you do?

A. From the Microsoft 365 admin center, add adatum.com as a custom domain name.
B. From Windows PowerShell, run the Set-ADDomain –AllowedDNSSuffixes adatum.com command.

C. From Active Directory Users and Computers, modify the UPN suffix of the five user accounts.

D. From the Microsoft 365 admin center, add adatum.local as a custom domain name.