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 refers to the current device used to access it. 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.