Final answer:
A proxy server is the most effective control for restricting access to unauthorized internet sites in an organization.
Step-by-step explanation:
The most effective control for restricting access to unauthorized internet sites in an organization is a proxy server. A proxy server acts as an intermediary between the organization's internal network and the internet, filtering and controlling web traffic. By configuring the proxy server to block unauthorized sites, the organization can restrict access to such sites for all devices connected to their network.
A firewall is also an important security measure, but its primary role is to monitor and block incoming and outgoing network traffic based on predefined rules. While a firewall can provide some level of access control, it is not as effective as a proxy server for restricting access to specific websites.
Antivirus software and intrusion detection systems are not specifically designed to restrict access to internet sites. Antivirus software is used to detect and remove malware from devices, while intrusion detection systems are used to monitor network traffic for suspicious activity.