Which service would be effective for providing authentication to multiple PaaS websites?

The most effective service for providing authentication to multiple PaaS (Platform as a Service) websites is federation services. This approach allows users to authenticate once and then access multiple applications or services without needing to log in again to each one. Federation services utilize identity providers (IdPs) to manage user identities across different platforms, streamlining the authentication process while ensuring security and a seamless user experience.

While multi-factor authentication enhances security by requiring an additional verification step beyond just a password, it does not inherently facilitate authentication across multiple platforms. Instead, it applies a layer of security to the existing authentication process.

Role-Based Access Control (RBAC) is primarily focused on managing permissions and access rights for users based on their roles within an organization. It does not serve as a mechanism to authenticate users across different services.

Mandatory access control (MAC) is a model that restricts access based on predetermined security levels of users and data. While it can be part of a security framework, it is not a service designed specifically for user authentication across multiple services.

In summary, federation services are designed specifically for scenarios involving multiple applications, enabling a centralized authentication method that improves user experience while maintaining high security across different PaaS offerings.