The correct answer is Single sign-on (SSO). SSO is an authentication scheme that allows a user to log in with one set of credentials to access multiple applications or systems. This reduces the number of prompts for credentials that the user must handle and simplifies account management for the administrator. The other options provided are different ways to authenticate users but do not inherently provide the streamlined multi-system access that SSO offers. Password vaults securely store multiple passwords, but they require interaction each time a different system is accessed. Furthermore, RADIUS is an authentication, authorization, and accounting protocol used in network access, not a solution for single sign-on across multiple systems. Security Assertion Markup Language (SAML) is a standard for exchanging authentication and authorization data between parties, specifically between an identity provider and a service provider, but it's not the authentication solution itself, rather it's a component of single sign-on implementations.