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:
If you go to the SharePoint Online Administration Center and click the “Add Domain” button it takes you to the same location as if you would have clicked the
“Domains” -> “Add domain” option from the Office 365 admin center.
So either A or B is technically correct, but if I had to choose one of the two I would select B.
https://support.office.com/en-us/article/Rename-your-SharePoint-Online-Public-Website-address-to-use-your-custom-domain-3D4BD288-772B-4F88- AF4DF025B3825ED3
https://support.office.com/en-us/article/Rename-your-SharePoint-Online-Public-Website-address-to-use-your-custom-domain-3403c6d5-aaa6-4775- a1ccc6bda0a99986?ui=en-US&rs=en-US&ad=US
https://support.office.com/en-us/article/Verify-your-domain-in-Office-365-6383f56d-3d09-4dcb-9b41-b5f5a5efd611?ui=en-US&rs=en-US&ad=US