The correct answer is D. an app registration. Microsoft Learn states that to delegate identity and access management functions to Microsoft Entra ID, an application must be registered with a Microsoft Entra tenant. When you register an application, you create its identity configuration in Microsoft Entra, and a corresponding service principal is created so the application can authenticate and integrate with the tenant. This is the standard Microsoft mechanism for allowing a third-party cloud service or app to authenticate to Microsoft Entra.