Which of the following is an authentication option when using PKI?

When using Public Key Infrastructure (PKI), the approach to authentication can involve multiple methods, including the use of both two-factor authentication and certificates. PKI provides a secure framework that relies on digital certificates issued by a trusted Certificate Authority (CA).

Two-factor authentication enhances security by requiring two forms of verification to prove the identity of the user. This could involve something they know (like a password) and something they have (such as a cryptographic token or a smart card that contains a certificate). By incorporating both two-factor authentication and certificates, this option provides a layered security model that significantly reduces the risk of unauthorized access.

The other options present more limited scopes of authentication. Some may suggest alternatives like single sign-on, which relies more heavily on a centralized authentication process rather than the distributed nature of PKI. Additionally, relying solely on usernames and passwords or just certificates may not offer sufficient security on their own, as they may not include the multifactor aspect that PKI can provide. Thus, option C, which embodies a robust method by integrating two-factor and certificates, accurately reflects the capabilities of PKI in enhancing secure authentication processes.