Explanation:

A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Its primary purpose is to establish a barrier between a trusted internal network and untrusted external networks, such as the internet, to prevent unauthorized access and protect against various cyber threats. Option (1) is incorrect; the physical boundary refers to hardware. Option (2) is incorrect; an operating system manages computer resources. Option (4) is incorrect; web browsing software accesses websites. Thus, only option (3) accurately defines a firewall's core function.