Dig is a command line tool that can be used in the reconnaissance phase of a network vulnerability assessment. Dig stands for domain information groper, and it is used to query Domain Name System (DNS) servers and obtain information about domains, hosts, and records. Dig can help discover the network topology, the IP addresses, and the services running on the target network.

B. ifconfig is a command line tool that can be used to configure and display the network interfaces on a system, but it is not useful for the reconnaissance phase of a network vulnerability assessment, as it only shows the local information and not the target information.

C. ipconfig is a command line tool that can be used to configure and display the network settings on a Windows system, but it is not useful for the reconnaissance phase of a network vulnerability assessment, as it only shows the local information and not the target information.

D. nbtstat is a command line tool that can be used to display the NetBIOS over TCP/IP statistics and information on a Windows system, but it is not useful for the reconnaissance phase of a network vulnerability assessment, as it only works for NetBIOS-enabled systems and not for other protocols.

References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 7, page 411; Official (ISC)2 CISSP CBK Reference, Fifth Edition, Chapter 7, page 365