You administer an application that has an Azure Active Directory (Azure AD) B2C tenant named
contosob2c.onmicrosoft.com.
Users must be able to use their existing Facebook account to sign in to the application.
You need to register Facebook with your Azure AD B2C tenant.
What should you do?

A.
a sign-up policy

B.
a custom user attribute

C.
a local account

D.
a built-in user attribute

E.
an identity provider