Which of the following defines an open standard for user authentication across co-operating sites?

Open ID is indeed defined as an open standard for user authentication across cooperating sites. It allows users to log into different services using a single set of credentials, which streamlines the login process and enhances user experience by eliminating the need to remember multiple usernames and passwords for each site. OpenID works by delegating the authentication process to a central provider that verifies the user's identity and shares that information with different applications, enabling seamless access.

In essence, OpenID provides a decentralized approach to user authentication, enabling users to maintain control over their identity without shedding security. This is particularly useful in web environments where numerous applications might require user verification and authorization.

Other options, while related to user authentication and security, serve different purposes. For instance, SAML is primarily an XML-based framework for exchanging authentication and authorization data between an identity provider and a service provider, but it is not classified as an open standard in the same context as OpenID. Similarly, Shibboleth is an open-source project based on SAML but focuses more on web single sign-on and federated identity solutions within educational and research institutions. LDAP is a protocol for accessing and maintaining distributed directory information services, which is more concerned with managing user data rather than the specific standard for user authentication across multiple