What is a part of a computer system designed to block unauthorized access?

A firewall is specifically designed to monitor and control incoming and outgoing network traffic based on predetermined security rules. Its primary function is to establish a barrier between a trusted internal network and untrusted external networks, such as the internet. Firewalls can prevent unauthorized access to or from a private network by filtering traffic based on various security rules, allowing legitimate traffic while blocking potentially harmful transmissions.

In contrast, routers are responsible for directing data packets between networks but do not inherently provide the security functions of a firewall. Antivirus software is designed to detect and eliminate malicious software but does not block access to networks. Switches primarily function to connect devices within a network, allowing them to communicate, but they do not offer security features against unauthorized access. Thus, the role of a firewall aligns perfectly with the task of blocking unauthorized access, making it the correct choice.