Which authentication method is primarily based on the physical attributes of the user?

The correct answer is biometric authentication, as this method relies on the unique physical characteristics of an individual to verify their identity. Biometric authentication encompasses traits such as fingerprints, facial recognition, retinal scans, and voice recognition. These attributes are inherent to the person and cannot be easily replicated or stolen, making biometric systems highly secure.

In contrast, password-based authentication involves knowledge that the user has (a secret string of characters), which can be forgotten or compromised. Hard tokens are physical devices that generate a code or contain cryptographic keys but don’t use physical characteristics of the user. Single sign-on simplifies user experience by allowing a single authentication process to grant access to multiple systems, but this method is not based on physical traits. Biometric authentication stands out precisely because it uses intrinsic physical data for identity verification.