Explanation:

A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Its primary and most direct purpose is to establish a barrier between a trusted internal network (like your PC) and untrusted external networks (like the internet) to prevent unauthorized access, block malicious data packets, and protect against various cyber threats. Therefore, 'Security' is the most accurate and encompassing function of a firewall. Option (b) 'Authentication' is the process of verifying a user's identity, which is a separate security mechanism, though firewalls can be part of a larger security architecture that includes authentication. Option (c) 'Data transmission' is the process of sending and receiving data; firewalls *regulate* and *control* data transmission for security purposes, but they are not primarily for the act of transmission itself. While a firewall contributes to overall system protection, its core role is specifically network security, making (a) the best answer.