What is an example of two-factor authentication?

Two-factor authentication (2FA) enhances security by requiring two different methods of verification to access a system or resource. The correct example of two-factor authentication is the combination of a Common Access Card (CAC) and a Personal Identification Number (PIN).

In this scenario, the CAC serves as a physical token that contains a digital certificate, providing something you have. The PIN represents something you know, which together fulfills the criteria for two-factor authentication. This method significantly strengthens security by ensuring that even if one factor (the PIN) is compromised, the physical token (the CAC) is still needed for access.

Other examples provided, such as a username and password, represent a single factor—something you know—and do not fulfill the two-factor requirement since they rely solely on knowledge. Options like a social security number and date of birth or a biometric fingerprint scan, while perhaps secure in their own contexts, do not combine two distinct forms of identification, thus failing to meet the criteria for two-factor authentication.