Established trust relationship is correct because federated identity requires the application and the external identity provider to have an agreed and trusted relationship before the application will accept assertions.

By Established trust relationship we mean that both parties must agree on identity metadata, issuer and audience values, and how assertions are validated. This agreement often includes exchanging signing keys or metadata and configuring the relying party so that signatures and claims can be verified. The trust is the conceptual requirement that lets the application decide which tokens or assertions are legitimate.

TLS is incorrect because while it secures the transport channel it does not by itself prove that the assertion was issued by a trusted identity provider. Transport security is complementary but it does not establish the federated trust.

Cloud Identity is incorrect because it is a product or service name and not the fundamental requirement for federation. You can establish a Established trust relationship with many different identity providers or solutions and you do not need a specific product to create the trust.

Public key certificates are incorrect as the primary answer because they are an implementation mechanism used to validate signatures and help implement trust. They are commonly part of how a trust relationship is realized but the underlying necessity on the exam is the actual Established trust relationship.

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