You have an Azure Active Directory (Azure AD) tenant named contoso.com that contains an Azure AD enterprise application named App1. A contractor uses the credentials of user1@outlook.com. You need to ensure that you can provide the contractor with access to App1. The contractor must be able to authenticate as user1@outlook.com. What should you do? 

Answer options:

A. Run the New-AzADUser cmdlet.
B. Configure the External collaboration settings.
C. Add a WS-Fed identity provider.
D. Create a guest user account in contoso.com.

D

Reference: https://docs.microsoft.com/en-us/azure/active-directory/external-identities/b2b-quickstart-add-guest-users-portal