The correct answer is SSO (Single Sign-On) because it enables a user to authenticate once and gain access to multiple systems or resources without being prompted to log in again. According to CompTIA Network+ (N10-009) security objectives, SSO improves usability and productivity while maintaining centralized authentication control. After the initial authentication, a trust relationship between systems allows the user to access additional applications seamlessly.

MFA (Multi-Factor Authentication) enhances security by requiring two or more authentication factors (something you know, have, or are), but it does not inherently provide access to multiple systems without repeated authentication events.

SAML (Security Assertion Markup Language) is an authentication and authorization protocol commonly used to implement SSO in web-based environments. While SAML supports SSO functionality, it is the underlying protocol rather than the access method itself.

RADIUS is a centralized authentication, authorization, and accounting (AAA) protocol used for network access control but does not specifically provide seamless multi-resource authentication without additional logins.

Therefore, SSO best describes authentication to multiple resources without requiring additional passwords.