You want to receive an email when the build fails. 
What should you do?

Answer options:

A.Create a new email template
B.Create a service hook
C.Create an agents pool
D.Create a new subscription
E.Create a new security group

Correct Answer: D 
Azure DevOps provides project notifications for the DevOps events like build, code, pipelines, work items, releases, etc. 
If you need to be notified by email when a build fails, you need to create a new subscription to the build events. You can do that from the Project settings (Number 1) and the Notifications (Number 2) menu item. Then you select the New subscription from the toolbar (Number 3). On the new screen, you choose an appropriate event type from the Category list (Number 4), e.g., Build, and a notification template, e.g., A build fails (Number 5). 

After you press the Next button, you need to provide a subscription description (Number 1) on the new screen, choose a delivery option like Team members by role, Team preference, Custom email address, etc. Select Custom email address (Number 2) and provide your email address (Number 3). Then add filter conditions, if needed, and push the Finish button. 

Azure DevOps will create a new notification subscription and add it to the project notifications list. 

All other options are incorrect.
For more information about project notifications, please visit the below URLs: 

https://docs.microsoft.com/en-us/azure/devops/notifications/manage-team-group-global-organization-notifications?view=azure-devops&tabs=new-account-enabled
https://docs.microsoft.com/en-us/azure/devops/notifications/manage-your-personal-notifications?view=azure-devops&tabs=new-account-enabled