Which security measure is designed to protect a network by blocking unauthorized access while permitting outward communication?

The correct choice is a firewall, which is specifically designed to safeguard a network by controlling inbound and outbound traffic based on predetermined security rules. Firewalls act as a barrier between a trusted internal network and untrusted external networks, such as the internet. They inspect the data packets that attempt to enter or exit the network and allow or block them based on the established security policies.

A firewall can help prevent unauthorized access to the network while still allowing users within the network to communicate outwardly with external systems. This dual functionality is crucial for maintaining both security and usability in network environments.

In contrast, an Intrusion Detection System is primarily focused on monitoring network traffic for suspicious activity and potential threats rather than actively blocking unauthorized access. A Virtual Private Network (VPN) secures remote access to a network by encrypting data transmitted over the internet but does not inherently control access to the network itself. A router, while essential for directing data between networks, does not manage security aspects, focusing instead on the forwarding of data packets.