Company A has acquired Company

Attestation is a process of verifying the integrity and authenticity of a system or component, often related to hardware security, and is not a mechanism for cross-organizational identity integration.

Federation establishes a trust relationship between two separate identity providers or organizations, allowing users from the acquired company to securely access resources in the acquiring company using their existing credentials without merging their directory services. This enables seamless cross-organizational identity management.

Single sign-on (SSO) allows a user to authenticate once to access multiple applications, typically within the same security domain, but it is a user experience feature that relies on an underlying identity solution like federation to work across organizations.

Kerberos is a network authentication protocol primarily used for secure authentication within a single domain or forest, not for establishing identity trust between two distinct and separate organizations.