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 its loopback network interface. Using the loopback interface bypasses any local network interface hardware.
The local loopback mechanism is useful for testing software during development, independently of any networking configurations. For example, if a computer has been configured to provide a website, directing a locally running web browser to http://localhost …