Which of the following techniques can be used for authenticating users?

Two-factor authentication is a widely recognized technique used for authenticating users because it enhances security by requiring two separate forms of verification before granting access. This method typically involves something the user knows (like a password) and something the user has (like a mobile device or a hardware token). By implementing two factors, it significantly reduces the risk of unauthorized access, even if one of the credentials is compromised.

In contrast, data encryption focuses on protecting the confidentiality of information at rest or in transit rather than confirming a user's identity. Intrusion detection systems monitor network traffic for suspicious activities, which is a security measure but does not directly authenticate users. Likewise, firewall monitoring helps to safeguard networks by controlling incoming and outgoing traffic based on predetermined security rules but again does not serve the purpose of authenticating users. Therefore, two-factor authentication stands out as the appropriate method for verifying user identities.