Which command can be issued at the following prompt?
Router(config-router)#

A.
show interface

B.
network

C.
interface

D.
ip default-gateway

Explanation:
The network command can be issued at the Router(config-router)# prompt, which also indicates that the router
is in router configuration mode. The network command is used to configure the network upon which a routing
protocol is functioning.
The router configuration mode is accessed by issuing the router command in the global configuration mode
along with a parameter indicating the routing protocol to be configured. For example:
R4(config)#router eigrp 1changes the prompt to:
R4(config-router)#
which then allows you to specify the network as follows:
R4(config-router)#network 192.18.5.0
All other options are incorrect as these commands can be issued only in the global configuration command
mode (which would be indicated by the R4(config)# prompt.
Objective:
Infrastructure Management
Sub-Objective:
Configure and verify initial device configuration

Cisco > Support > Cisco IOS Software > Using the Command-Line Interface in Cisco IOS Software