Which of the following components splits a computer network into subnetworks in order to boost performance and improve security?

A.
Monitoring

B.
Fragmentation

C.
WLAN controller

D.
Network segmentation

Explanation:
Network segmentation in computer networking is the act or profession of splitting a computer network into subnetworks, each being a network segment or network layer. The advantages of such splitting are primarily for boosting performance and improving security.
Advantages:

Reduced congestion: Improved performance is achieved because on a segmented network, there are fewer hosts per subnetwork, thus minimizing local traffic. Improved security: Broadcasts will be contained to the local network. Internal network structure will not be visible from outside.
Containing network problems: It limits the effect of local failures on other parts of the network. Answer option B is incorrect. Fragmentation occurs when parts of a file become scattered in different areas of a hard disk. It occurs when users create, delete, and modify files. When the parts of a file are scattered on the hard disk, the operating system searches for all the parts of the file, which are stored on different locations of the hard disk, to access the file. This process slows down the overall performance of the computer.
Answer option C is incorrect. A wireless LAN controller is a device that is used in combination with Lightweight Access Point Protocol (LWAPP) to manage light weight access points in large quantities by the network administrator or NOC. The Wireless LAN controller is part of the Data Plane within the Cisco Wireless Model. The WLAN controller automatically handles the configuration of anywhere from 6 to 300 wireless access-points, depending on the model. Answer option A is incorrect. Monitoring is one of the components required for securing a wireless 802.11 network. Monitoring is done in order to prevent possible attacks against a WLAN. It is important to monitor the wireless network once it is installed and designed. For proper monitoring, wireless intrusion detection system (WIDS) and wireless intrusion prevention system (WIPS) are used. Both WIDSs and WIPSs have the ability to detect valid and invalid devices on the network.