A company wants to move all its employees to Azure Active Directory. What is the initial domain where the tenant is created?

Answer options:

A.onmicrosoft.com
B.portal.azure.com
C.azurewebsites.com
D.microsoft.com

Answer: A 
Option A is correct as is used for the initial domain name should be onmicrosoft.com and Also, even the diagram included in the explanation indicates the initial domain is onmicrosoft.com. 
Option B is incorrect as portal.azure.com is the used for login into the Azure portal. 
Option C is incorrect as azurewebsites.com is used to host the web application in azure. 

http://docs.microsoft.com/en-us/azure/active-directory/fundamentals/active-directory-whatis

Answer: A 
Option A is correct as is used for the initial domain name should be onmicrosoft.com and Also, even the diagram included in the explanation indicates the initial domain is onmicrosoft.com. 
Option B is incorrect as portal.azure.com is the used for login into the Azure portal. 
Option C is incorrect as azurewebsites.com is used to host the web application in azure. 
Diagram: