Which of the following protocols is related to mutual authentication through credentials?

The selection of EAP-FAST as the correct answer highlights the protocol's mechanism for mutual authentication between the client and server using credentials. EAP-FAST, which stands for Extensible Authentication Protocol - Flexible Authentication via Secure Tunneling, establishes a secure tunnel and uses credentials to ensure that both parties can authenticate each other. This is integral in scenarios such as wireless networks, where verifying both the user's identity and the authenticator’s identity is critical to maintaining security.

In contrast, EAP-TLS, while also providing strong security through mutual authentication using digital certificates, requires a Public Key Infrastructure (PKI), which may not be as flexible as EAP-FAST in all deployment scenarios. Diameter is a protocol used for authentication, authorization, and accounting, but isn't specifically centered around mutual authentication through credentials in the same way as EAP-FAST. Single Sign-On (SSO) facilitates user access to multiple services with one set of credentials but does not inherently provide a framework for mutual authentication, focusing more on user convenience rather than two-way verification.

The emphasis on credential-based mutual authentication in EAP-FAST makes it the most relevant choice in the context of the question.