Prominent recommended standards to enable federation of identity in cloud environments include:

FIDO is a standard for passwordless and multi-factor authentication at the device level, not for federating identity assertions between separate organizational domains.

Kerberos is a ticket-based network authentication protocol designed for use within a single Kerberos realm or trusted domain and does not natively support cross-domain cloud federation.

SSO is a user experience outcome and architectural goal, not a protocol or standard - it describes what federation achieves rather than the mechanism used to achieve it.

SAML (Security Assertion Markup Language) is an XML-based OASIS standard specifically designed for cross-domain identity federation, allowing an identity provider to pass signed authentication and authorization assertions to a service provider - making it the recommended protocol for enabling SSO and identity federation across cloud environments without sharing credentials.

X.509 defines the format for public key certificates used in PKI and TLS, but it is not a federation protocol and does not handle the exchange of identity assertions between providers.