Under user authentication, which factor sets may be used to authenticate all users?

• A. Something you know, something you have, or something you are.
• B. Something you know only.
• C. Something you have only.
• D. Something you are only.

Answer: (A)

Authentication relies on three independent factor types: something you know (like a password), something you have (such as a token or smart card), and something you are (biometrics). The question asks which factor sets may be used to authenticate all users, and the valid answer is the union of these categories. This reflects that a user can be verified by any one of these factors or by a combination of them, depending on the security requirements. In practice, many systems employ two or more factors for stronger protection, especially for remote or privileged access, but the fundamental options include all three categories. The other choices constrain authentication to a single factor type, which isn’t generally how authentication is designed to work for all users.