Melania's organization is part of a federation that allows users from multiple organizations to access resources and services at other federated sites. When Melania wants to use a service at a partner site, which identity provider is used?

In federated systems, a user's access to services is authenticated through their own organization's identity provider (IDP). Service providers query those IDPs when the user attempts to authenticate to the service and, if the request is validated, allow access based on the rules and policies set for the service based on attributes that may be relevant that are provided by the IDP.