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?

Role Based Access Control (RBAC) is an authorization model that assigns permissions based on a user's assigned role, not a mechanism for centralizing authentication or eliminating repeated logins across systems.

Discretionary Access Control (DAC) lets individual resource owners define who may access their resources, but it has no concept of a central authentication server or single-login capability.

Windows authentication refers to specific protocols like NTLM or Kerberos within a Windows domain and is not a general-purpose multi-system single-login mechanism applicable across heterogeneous environments.

Single sign-on implements a central authentication server (CAS) where a user provides credentials once and receives a token or session that is trusted by all connected systems. This federates identity across applications so the user never re-authenticates per system. The CAS is the authoritative validator, and downstream services accept its proof of identity rather than managing their own credential stores.