You need to configure Office 365 to simplify the management of user identities

You are an IT administrator for Contoso, Ltd. You have subscribed to Microsoft Office 365.
You are preparing to set up your domain. Your company already uses the custom domain name
contoso.com for other cloud services.
You need to configure Office 365 to simplify the management of user identities.
What should you do before you create new user accounts?

A.
Create a new custom domain name specifically for Office 365.

B.
Configure Office 365 to use your custom domain

C.
Verify that the automatic domain contoso.onmicrosoft.com is functioning

D.
Add the .onmicrosoft suffix to your custom domain name.

Explanation:
Custom domains
To use your own domain name with Office 365 instead of the domain name that you were given at
signup, you add the domain to Office 365.
https://technet.microsoft.com/en-us/library/office-365-domains.aspx