Which of the following is the MOST secure way to mutually authenticate an external server to an internal server for a data exchange?

While tickets like those used in Kerberos provide authentication, they are primarily for single sign-on within a trusted domain and do not provide the same level of secure mutual authentication for arbitrary external/internal servers as PKI certificates.

A VPN establishes a secure tunnel for communication but is not the primary mechanism for mutual authentication itself; it can use certificates for authentication, but the certificate is the core component that enables the mutual authentication.

IPSec secures communication at the network layer and can employ certificates for authentication, but certificates are the underlying technology for establishing the mutual trust and identity verification.

Certificates, leveraging Public Key Infrastructure (PKI), provide a robust and scalable method for mutual authentication by allowing each server to cryptographically prove its identity to the other. During the authentication process, each server presents its digital certificate, which contains its public key and is signed by a trusted Certificate Authority (CA), allowing the other server to verify its authenticity and establish trust before data exchange.