Which authentication standard is most effective when integrating multiple external service providers?

The appropriate choice is SAML (Security Assertion Markup Language) because it is specifically designed for enabling single sign-on (SSO) and identity federation across different domains. SAML allows for the exchange of authentication and authorization data between an identity provider and a service provider, making it particularly effective in scenarios where multiple external service providers are involved.

Using SAML, organizations can centralize their authentication processes. This means that users can log in once to an identity provider, and through SAML, that authentication can be recognized by multiple service providers without needing to log in separately to each one. This reduces the burden on users to manage multiple credentials and simplifies the user experience.

Moreover, SAML's reliance on XML and its support for a wide array of use cases—involving cloud services, enterprise systems, and web applications—further broadens its applicability in diverse environments where external service integration is necessary. This standard promotes better security practices by allowing organizations to enforce policies centrally, making it easier to manage authentication across various systems and services.