You need to set up Slack channel notification when a Pull request is created in Azure Repo. 
What should you do to achieve your goal?

Answer options:

A.Create a service principal
B.Enable third-party application access via OAuth
C.Enable Allow public projects
D.Create a service hook
E.Create a new security group

Correct Answer: D 
Azure DevOps integrates with several Microsoft and third-party applications like GitHub, Jira, Teams, Slack, ServiceNow, etc. 
Slack is an enterprise communication platform that provides tools for effective team collaboration like chat rooms organized by topics (channels), private groups, direct messaging, file sharing, etc. 
You can integrate Azure DevOps with Slack to notify the team members about different DevOps events in Repos, Boards, or Pipelines. Azure DevOps will post messages to Slack channels when an appropriate event is triggered. 

All other options are incorrect.
For more information about service hooks integration and service hooks for Slack, please visit the below URLs: 

https://docs.microsoft.com/en-us/azure/devops/service-hooks/services/slack?view=azure-devops
https://docs.microsoft.com/en-us/azure/devops/service-hooks/overview?view=azure-devops