What security measure improves the robustness of an access control method?

Implementing two-factor authentication significantly enhances the robustness of an access control method by adding an additional layer of security beyond just a username and password. This method requires users to provide two different forms of verification before accessing a system, which typically includes something they know (like a password) and something they have (like a smartphone app that generates a code or a hardware token).

This dual verification process makes unauthorized access much more difficult since even if an attacker manages to obtain the password, they would still need the second factor to gain entry. This added complexity can help prevent data breaches and ensure that only authorized users can access sensitive information or systems.

The other options do not contribute to stronger security practices. For instance, using a single password for all users can lead to significant vulnerabilities, as any breach could potentially compromise all accounts linked to that password. Allowing unlimited access to all users bypasses the fundamental principles of access control, leading to a lack of accountability and oversight. Employing outdated credential types further exposes systems to risks, as they may not comply with current security standards and could be easily exploited by attackers.