Which of the following describes a multi-factor authentication system?

Multi-factor authentication (MFA) is a security mechanism that requires users to provide two or more verification factors to gain access to a system, enhancing overall security. The correct answer involves a password combined with a code generated by a mobile application, which exemplifies the principle of using different types of authentication factors: something the user knows (the password) and something the user has (the code from the mobile application).

This combination significantly reduces the risk of unauthorized access, as even if an attacker acquires the password, they would still need access to the mobile device generating the code to successfully log in.

In contrast, the other options represent single-factor or dual-factor methods that do not sufficiently address the need for MFA. A username and password combination, while common, only utilizes one authentication factor. Similarly, a challenge question alongside a password does not provide a sufficient barrier against potential security breaches, as both factors are still based on something the user knows. Lastly, a single password entry alone is the least secure, as it does not incorporate any additional verification, leaving user accounts vulnerable to attacks.