Which system is responsible for user authentication for Google+ users?

The responsibility for user authentication for Google+ users lies with Google's servers. This is because user authentication involves verifying credentials such as usernames and passwords to confirm a user's identity before granting access to services. Google has developed a comprehensive authentication mechanism that manages user accounts and securely verifies credentials across its various platforms, including Google+.

Using Google’s servers for authentication ensures that sensitive information is handled securely and efficiently. They implement strong security measures, including multi-factor authentication, to protect user accounts from unauthorized access. This centralizes the authentication process and allows for better management of user identities, ensuring consistency and security across Google's services.

In contrast, while the user's local machine and the e-commerce application server may play roles in identity management and access implementation, they do not handle the primary authentication of Google+ users. The privacy policy of the e-commerce site does not deal with the technical aspects of authentication but rather addresses how user data is handled and protected. Thus, Google's servers are the correct answer, as they are directly responsible for authenticating and managing user identities for services like Google+.