You need to change the address of the SharePoint Online…

Your company uses Office 365 and has an Enterprise E3 plan. The company has a Microsoft SharePoint
Online public website that is currently configured to use the onmicrosoft.com domain name.
The company purchases a new domain name.
You need to change the address of the SharePoint Online public website to the new domain name.
What should you do first?

A.
In the SharePoint Online Administration Center, add the new domain.

B.
In the Office 365 admin center, add the new domain.

C.
Create a new site collection and assign it the new domain.

D.
Create a new public website and assign it to the new domain.

Explanation:
When you purchase an Office 365 subscription and configure a Microsoft SharePoint Online public website, the
website will use a URL like contoso.onmicrosoft.com.
If you want to use your own domain name, for example www.contoso.com, the first step is to add the domain in
the Office 365 admin center. This will start a wizard which will give you the option to create a new website using
the new domain name or assign the new domain name to an existing website. The last step in the wizard will
tell you which DNS records you need to configure to point the new domain name to the SharePoint Online
public website.
https://support.office.com/en-us/article/Rename-your-SharePoint-Online-Public-Website-addressto-use-your-custom-domain-3403c6d5-aaa6-4775-a1cc-c6bda0a99986?ui=en-US&rs=en-US&ad=US