Which identity proofing method involves a combination of two factors, such as something a user knows and something a user has, for authentication?

The correct answer is Two-Factor Authentication (2FA), which is a security process that requires a user to provide two different forms of identification before accessing an account. This method enhances the security of authentication by requiring something the user knows, such as a password or PIN, paired with something the user has, like a smartphone or hardware token. The combination of these two factors significantly reduces the likelihood of unauthorized access since a potential attacker would need to obtain both pieces of information to gain access.

In contrast, other options represent different authentication or networking methods, which do not fit the specific criteria of combining two distinct factors. Diameter, for instance, is a protocol used for authentication, authorization, and accounting, but it doesn't inherently describe an identity proofing method. Time-Based One-Time Password (TOTP) is an authentication mechanism that generates one-time, time-sensitive codes, often used in conjunction with other methods, but it doesn't represent the two-factor concept itself. Out-of-band mechanisms provide an additional layer of security by using a separate communication channel to verify an action, but again, they may not directly define the traditional concept of two-factor authentication as distinct from single-factor authentication options.