Which of the following is commonly implemented in external devices to provide unique event- based two-factor authentication credentials when a system is already implementing username and password auth

TACACS+ is a network AAA protocol used for centralizing device access control, not a mechanism for generating second-factor credentials on external hardware devices.

HOTP is the algorithm standard for generating event-based one-time passwords, but it describes the cryptographic method rather than the user-facing credential entered on an external device.

Biometrics authenticate via physical characteristics and are not event-based credentials generated or entered through an external device.

LDAP is a directory access protocol for querying user and credential information, not a second-factor credential mechanism.

A PIN is commonly required to unlock an external authentication device such as a smart card or hardware token, serving as the second factor beyond the existing username and password. When tied to a physical external device, the PIN is unique to that device and is triggered as part of the authentication event, satisfying the two-factor requirement with something the user knows plus something the user has.