Which command must be entered to configure a DHCP relay?

`ip dhcp relay` is not a standard Cisco IOS command for configuring a DHCP relay agent on an interface; the correct command is `ip helper-address`.

`ip dhcp pool` is used to configure a DHCP server on the router itself, defining the range of IP addresses and other options, not to relay client requests to an external server.

`ip address dhcp` is an interface command used to configure the interface itself to obtain its IP address via DHCP, not to relay DHCP requests from other devices.

The `ip helper-address` command is configured on a router interface to forward specific UDP broadcast services, most notably DHCP (port 67 and 68), from a client's subnet to a DHCP server located on a different subnet. This enables DHCP clients to obtain IP addresses even if their DHCP server is not on the same local network segment.