Unapproved Device Warning

D.

The network appliance that warns of unapproved devices that are accessing the network is an Intrusion Detection System (IDS).

An IDS is a security tool that monitors network traffic for signs of malicious activity or policy violations. It operates by examining network packets and comparing them to a set of known attack signatures or behavioral patterns. If an IDS detects any suspicious activity, it generates an alert or notification to the network administrator, warning of a potential security breach.

In the context of this question, the IDS is being used to identify unapproved devices that are attempting to access the network. This can be useful in preventing unauthorized access and protecting sensitive data. An IDS can also help detect other types of attacks, such as denial-of-service attacks, malware infections, and network reconnaissance.

The other options listed in the question are also network appliances commonly used in network security, but they do not specifically address the issue of identifying unapproved devices accessing the network.

A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on a set of security rules. It can prevent unauthorized access and protect against network-based attacks, but it does not have the same level of visibility into network traffic as an IDS.

An AP (Access Point) is a device that enables wireless devices to connect to a wired network. It is not specifically designed to detect unapproved devices accessing the network.

A proxy server is a server that acts as an intermediary between a client and a server. It can be used to enhance security by filtering web content and blocking access to unauthorized websites, but it does not have the same level of visibility into network traffic as an IDS.

Therefore, the best answer to the question is D. IDS.