Which of the following is a secret numeric password used for authentication?

A personal identification number (PIN) is specifically a numeric password employed for authentication purposes. It is often used in conjunction with a physical card, such as a debit or credit card, to verify the identity of the user attempting to access a system or perform a transaction. The uniqueness of a PIN lies in its design: it is a confidential number that users must keep secret and only share through secure means.

In contrast, a security token provides an additional layer of security but is not solely a numeric password; it can be a hardware device, a software application, or even a part of a two-factor authentication process. A digital certificate, meanwhile, is used to establish the identity of an entity and is more complex, often involving cryptographic key pairs for secure communication, rather than a simple password. Lastly, a digital signature serves to confirm the authenticity and integrity of a digital message or document but does not function as a password. Therefore, the PIN stands out as the only choice that fits the criteria of being a secret numeric password specifically designed for authentication.