DSL and Cable modems are used in remote-access WAN technology for connecting to the Internet. Both provide an "always on" Internet access service.
Answer options C and A are incorrect. Analog and Digital modems are not always in 'ON' mode when connecting to an ISP. Analog modems transmit analog voice signals, while Digital modems transmit digital signals over a link.

A stateful firewall is a firewall that keeps track of the state of network connections (such as TCP streams, UDP communication) traveling across it. The firewall is programmed to distinguish legitimate packets for different types of connections. Only packets matching a known connection state will be allowed by the firewall; others will be rejected.Answer option B is incorrect. A stateless packet filter firewall allows direct connections from the external network to hosts on the internal network and is included with router configuration software or with Open Source operating systems. Answer option C is incorrect. It applies security mechanisms when a TCP or UDP connection is established. Answer option D is incorrect. An application gateway firewall applies security mechanisms to specific applications, such as FTP and Telnet servers.

Nmap
Explanation:
Nmap is a free open-source utility for network exploration and security auditing. It is used to discover computers and services on a computer network, thus creating a "map" of the network. Just like many simple port scanners, Nmap is capable of discovering passive services. In addition, Nmap may be able to determine various details about the remote computers. These include operating system, device type, uptime, software product used to run a service, exact version number of that product, presence of some firewall techniques and, on a local area network, even vendor of the remote network card. Nmap runs on Linux, Microsoft Windows, etc.