Explanation
The best recommendation is to place an intrusion detection system (IDS) between the firewall and the Internet.
An IDS is a device or software that monitors network traffic for malicious activity and alerts the network administrator or takes preventive action. By placing an IDS between the firewall and the Internet, the IS auditor can enhance the security of the network perimeter and detect any attack attempts that the firewall was unable to recognize.
The other options are not as effective as placing an IDS between the firewall and the Internet:
Placing an IDS between the firewall and the organization's web server would not protect the web server from external attacks that bypass the firewall. The web server should be placed in a demilitarized zone (DMZ), which is a separate network segment that isolates public-facing servers from the internal network.
Placing an IDS between the firewall and the demilitarized zone (DMZ) would not protect the DMZ from external attacks that bypass the firewall. The DMZ should be protected by two firewalls, one facing the Internet and one facing the internal network, with an IDS monitoring both sides of each firewall.
Placing an IDS between the firewall and the organization's network would not protect the organization's network from external attacks that bypass the firewall. The organization's network should be protected by a firewall that blocks unauthorized traffic from entering or leaving the network, with an IDS monitoring both sides of the firewall.