Which of the following is NOT a function of a firewall?

The role of a firewall is primarily focused on monitoring and controlling incoming and outgoing network traffic based on predetermined security rules. Firewalls are key components of network security, functioning as barriers between trusted internal networks and untrusted external networks, filtering traffic to protect against unauthorized access and threats.

Filtering incoming traffic for security is a fundamental function of a firewall, as it screens and blocks potentially harmful data packets while allowing legitimate traffic to pass through. Regulating traffic between different networks is also a crucial function, as firewalls can create rules that govern how data flows between internal and external networks or between different segments of a corporate network.

In contrast, performing monetary transactions on behalf of users is not a function of a firewall. This activity falls under the domain of payment processing systems and financial transaction services, which implement their own sets of security measures but are not part of a firewall's capabilities. Similarly, providing log-in services for authorized users involves user authentication processes that are typically managed by access control systems or application servers, rather than the firewall itself.

Therefore, option C is the correct answer as it identifies a function that does not align with the purpose and operations of a firewall.