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
Wikipedia
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