Answer D is correct because 127.0.0.1 is the IPv4 loopback address. Answer A is a mac address, while B and C are IPv6 addresses, in IPv6 the loopback is ::1
In computer networking, localhost is a hostname that means this computer. It is used to access the network services that are running on the host via the loopback network interface. Using the loopback interface bypasses any local network interface hardware.
The local loopback mechanism may be used to run a network service on a host without requiring a physical network interface, or without making the service accessible from the networks the computer may be connected to. …