Which of the following service in Azure can be used to enable Multi-Factor authentication for Azure users?

Answer options:

A.Azure Sentinel
B.Azure Key Vault
C.Azure Security Center
D.Azure Active Directory

Answer : D 
There are a couple of ways to enable Azure Multi-Factor Authentication. 
This can be done from Azure Active Directory 
As shown below , if you go to the Users section, you can enable Multi-Factor Authentication 

You can’t enable Multi-Factor Authentication in the other services, that is why the other options are incorrect. 
For more information on Azure Multi-Factor Authentication, please visit the below URL 

https://docs.microsoft.com/en-us/azure/active-directory/authentication/concept-mfa-howitworks