A firewall can hide computer systems as well as the ports from invaders. Therefore, it is the correct choice. Option (A): It is used for monitoring the traffic over the internet. It checks the flow of packets across the systems over the internet. This makes it a wrong choice. Option (C): All the packets over the network are checked like the ports, IP addresses, type of protocols etcetera help in either blocking or passing the packet, but it does not hide the system. As a result, this is an incorrect choice. Option (D): It protects the computer system from letting malware harm them. It does not block the port or hide the system, all of this makes it not a required choice.