Which of the following is not a recognized authentication factor?

Single Sign-On (SSO) is a system that allows users to log in once and gain access to multiple applications or services without needing to authenticate for each one separately. While SSO streamlines the user experience by centralizing the authentication process, it does not represent an authentication factor in itself.

Authentication factors are typically categorized into three main types: something you know (like a username and password), something you have (such as a token or smart card), and something you are (like biometric data). SSO functions using these existing factors rather than being a distinct method or type of an authentication factor. Therefore, when evaluating the options provided, SSO stands out as a mechanism for managing access rather than a pure authentication factor.