The US government CAC is an example of which type of authentication factor?

The correct answer is a smart card. A Common Access Card (CAC) used by the U.S. government is specifically designed to provide secure access to government facilities and information systems. It contains an embedded microchip that enables cryptographic functions, allowing it to serve as a portable, secure form of identification and authentication.

Smart cards function as physical tokens that store secure information, making them effective for two-factor authentication when combined with something you know, like a password, or something you are, like a biometric scan. In this case, the CAC provides a robust method of verifying the identity of its users through the smart card technology it employs.

Other options include various methods of authentication, each serving different roles. A password is a knowledge factor, while a biometric scan represents a physiological or behavioral characteristic. A token device refers to a physical device that generates a unique code for authentication, which is distinct from the smart card functionality.