What would be considered a multi-factor authentication method in a non-digital setting?

Multi-factor authentication refers to a security process that requires more than one form of verification to grant access to a system or physical space. In a non-digital setting, this means combining different types of security measures.

The correct choice exemplifies this concept by requiring two separate authentication methods for two doors: an outer door that requires a key and an inner door that needs a pin code. This setup employs two distinct factors for authentication: something the user has (the key) and something the user knows (the pin code). This layered approach significantly enhances security because even if one factor is compromised, unauthorized access remains prevented by the other factor.

In contrast, relying solely on a key, as in the first option, does not qualify as multi-factor authentication since it relies on a single form of verification. The third option similarly features just one method (the password lock) without an additional element for verification. Lastly, having security guards alone does not employ any method of authentication but rather provides a physical deterrent without layering any verification mechanics. Thus, the combination of distinct factors in the correct choice strengthens security measures in that scenario.