Which of the following is considered a two-factor authentication system?

Two-factor authentication (2FA) enhances security by requiring two different forms of identification from the user to gain access to an account or system. This approach helps ensure that even if one form of authentication is compromised, unauthorized access can still be prevented.

The combination of password and swipe card represents a two-factor authentication system because it involves something the user knows (the password) and something the user has (the swipe card). This dual approach adds an extra layer of security, requiring both elements to authenticate successfully.

In contrast, a user ID with a password, a user ID with a voice scan, and a password with a PIN all rely on one factor: either knowledge (like a password or PIN) or biometric traits (like a voice scan). These do not meet the criteria for two-factor authentication since they lack the requirement of presenting something in addition to what the user knows or is. Thus, option C is correctly identified as an example of a two-factor authentication system.