You are the IT administrator of a company with an on-premise Active Directory domain named teststraining.local and an Office 365 tenant. 
Your own the domain teststraining.com, and you have added it to Office 365 as the primary domain. Your Exchange solution is on premise, with the configured SMTP format user.name@teststraining.com. Your users UPN (User Principal Name) format is user.name@teststraining.local. 
You now want to install AD Connect on a member server and synchronize your users to Office 365. 
What should you do to prepare for the synchronization?

Answer options:

A.Make sure the AD connect agent is installed on an isolated server in a DMZ zone.
B.Group all users that should be synchronized in one Organizational Unit
C.Change the UPN (User Principal Name) for all users to match their SMTP address.
D.Add teststraining.com as a new UPN suffix in Active Directory Domains and Trusts.

Correct Answer: D 
teststraining.local is a non-routable domain name, which left unchanged will synchronize your users to an .onmicrosoft.com domain. You should therefore add Whizlabstraining.com as a new UPN Suffix by opening Active Directory Domains and Trusts, right-click Active Directory Domains and Trusts, and choose Properties and Add UPN Suffix: 

After you have added the new UPN suffix, you can change your users UPN from @teststraining.local to @teststraining.com in the user properties in Active Directory users and computers.
Option A is incorrect. There is no requirement for the AD connect agent to be installed in a DMZ zone.
Option B is incorrect. There is no need to group all users in one Organizational Unit. When configuring the AD Connect installation, you can choose which OU’s to synchronize (one or several).
Option C is incorrect. You should change the UPN to match your users SMTP addresses, but you must first add teststraining.com as a new UPN suffix under Active Directory Domains and Trusts. 
Reference:
To know more about preparing a non-routable domain for synchronization, please refer to the link below: 

https://docs.microsoft.com/en-us/microsoft-365/enterprise/prepare-a-non-routable-domain-for-directory-synchronization?view=o365-worldwide