Which of the following describes an example of single-factor authentication?

Single-factor authentication refers to a security process where only one type of authentication credential is used to verify a user's identity. This can involve something the user knows, such as a password or PIN, or something the user has, like a hardware token.

In the context of the provided options, the pairing of a username and password effectively constitutes single-factor authentication because it relies solely on the knowledge factor: the user must know both their username and the corresponding password to gain access. While the username serves as an identifier, the password is the key credential that confirms the user's identity.

The other options involve additional factors of authentication. For instance, a fingerprint requires a biometric factor, which adds a second layer, and similarly, the combination of a username, password, and PIN merges multiple elements, going beyond a singular point of authentication. Therefore, the correct identification of single-factor authentication is clearly represented by the requirement of only a password.