Which access control mechanism allows for multiple systems to use a central authentication server (CAS) that permits users to authenticate once and gain access to multiple systems?

SSO uses a Central Authentication Server to issue tokens or session credentials after a single login event, which are then accepted by all integrated downstream systems. This reduces credential fatigue, centralizes authentication policy enforcement, and provides seamless access across services - exactly matching the described scenario of authenticating once to gain access to multiple systems.

Windows Authentication is a specific protocol set (NTLM and Kerberos) tied to Microsoft environments and does not describe the general concept of a vendor-neutral CAS enabling multi-system access.

Role Based Access Control (RBAC) is an authorization model that assigns permissions based on job roles - it governs what users can do after authentication, not how they authenticate.

Discretionary Access Control (DAC) is an access control model where resource owners define permissions on their objects - it is an authorization concept unrelated to centralized authentication.