Just-in-Time (JIT) provisioning is a feature that allows Salesforce to create or update user records on the fly when users log in through an external identity provider. This eliminates the need for manual or batch user provisioning in Salesforce. References: Just-in-Time Provisioning for SAML and OpenID Connect, Identity
101: Design Patterns for Access Management

To provision users in Salesforce from Active Directory without doing any initial setup ofusers in Salesforce, UC can use an app exchange product or Identity Connect. An app exchange product is a third-party applicationthat can synchronize users and groups from Active Directory to Salesforce using a web-based interface1. Identity Connect is a desktop application that cansynchronize users and groups from Active Directory to Salesforce using a graphical user interface2. Both solutions can also map Active Directory attributes to Salesforce fields and assign profiles, roles, and permission sets to users12.
References: Active Directory Integration with Salesforce, Identity Connect

To meet the requirements of using a central cloud-basedIAM service for authentication and user management, the IAM architect should implement Salesforce Sales Cloud as a SAML service provider and enable SCIM for provisioning and deprovisioning of users. SAML is a protocol that allows users to authenticate andauthorize with an external identity provider and access Salesforce resources. By configuring Salesforce as a SAML service provider, the IAM architect can use the central IAM service as an identity provider and enable single sign-on for users. SCIM is a standard that defines how to manage user identities across different systems. By enabling SCIM in Salesforce, the IAM architect can synchronize user data between the central IAM service and Salesforce and automate user provisioning and deprovisioning based onthe changes made in the central IAM service. References: SAML Single Sign-On Settings, SCIM User Provisioning for Connected Apps

If the third-party authentication provider supports only the OAuth protocol and not OpenID Connect,then an identity architect needs to create a custom external authentication provider for it. A custom external authentication provider is a configuration that allows users to log in to Salesforce using an external identity provider that is not predefined by Salesforce. It requires implementing the Auth.AuthProviderPlugin interface and defining the OAuth endpoints and parameters.
References: Custom External Authentication Providers, Create a Custom Authentication Provider

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]