D is correct because Apex registration handler is the portion of the authentication provider setup that associates a Facebook user with a Salesforce user when customers use their Facebookcredentials to log in to the customer community. Apex registration handler is an Apex class that handles the logic for creating or updating a user record based on the information received from Facebook. A is incorrect because consumer key and consumer secret are portions of the authentication provider setup that identify and authenticate UC’s customer community with Facebook, not associate a Facebook user with a Salesforce user. B is incorrect because Federation ID is an attribute that can be used to identify a user in a SAML assertion when UC uses SAML-based SSO with Facebook, not when UC uses social sign-on with Facebook. C is incorrect because user info endpoint URL is a portion of the authentication provider setup that specifies the URL to obtain the user information from Facebook, not associate a Facebook user with a Salesforce user. Verified References: [Apex Registration Handler], [Consumer Key and Secret], [Federation ID], [User Info Endpoint URL]