Which of the following relies on a physical characteristic of the user to verify his identity?

Kerberos v5 is a network authentication protocol that uses ticket-based cryptographic tokens and shared secrets to verify identity, not any physical characteristic of the user.

Social engineering is a manipulation technique that exploits human psychology to deceive people into revealing information or granting access - it is an attack vector, not an authentication method.

CHAP (Challenge Handshake Authentication Protocol) verifies identity through a cryptographic challenge-response exchange using a shared secret password, not a physical characteristic.

Biometrics uses measurable physical traits - such as fingerprints, iris patterns, facial geometry, or voice recognition - to uniquely identify and verify a person. Because these characteristics are intrinsic to the individual, biometrics is classified as a 'something you are' authentication factor, directly tying identity verification to the physical user.