For social sign-on into Experience Cloud, Salesforce relies on authentication providers to establish trust with each social identity source and on a registration handler to create or update the Salesforce user and related records. That pattern applies whether the provider is Facebook, LinkedIn, Twitter, or a standards-based OpenID Connect provider. Process Builder or Flow does not replace the registration handler in this identity handshake. The registration handler is where Salesforce decides how to map the incoming identity to an existing user, when to create a new one, and how to update profile information over time. From an architecture standpoint, authentication providers solve the external trust, and the registration handler solves the user lifecycle inside Salesforce. This is why option C is the best answer in Salesforce terms.