For securing mobile devices with two authentication methods, combining something the user knows (a PIN) with something the user is (biometrics) enhances security by implementing multi-factor authentication.
Smart card: Generally requires additional hardware and is not commonly used in mobile devices.
Biometrics: Uses unique biological traits such as fingerprints or facial recognition, providing a convenient and secure second method of authentication.
Hard token: Involves additional physical devices which might not be practical for mobile devices.
One-time password: Usually used for specific applications rather than as a general device authentication method.