After Ann, a user, logs into her banking websites she has access to her financial institution mortgage, credit card, and brokerage websites as well. Which of the following is being described?

A trusted OS refers to an operating system built to meet formal security evaluation criteria (such as Common Criteria), not to cross-application authentication.

Mandatory access control (MAC) is an OS-level policy where access to resources is determined by labels and security clearances, not by session federation between websites.

Separation of duties is a policy requiring that no single individual can complete a sensitive process alone, which is unrelated to the authentication flow described.

Single sign-on (SSO) is an authentication mechanism that allows a user to log in once and gain access to multiple related but independent systems without re-entering credentials for each one. The federated trust between the banking portal and the affiliated financial services sites is the classic SSO use case.