In all the options given, the device will have a unique host as the admin of router maintains a mapping table.. which maintains the device's assigned IP with the device's MAC address.
(In IPV4, each computer sends a request packet to the router before establishing connection and hence the router maintains a mapping table of assigned IPs)
Now, the biggest network is Internet.
(The other answers given here say that D is correct due to unique MAC addresses in ethernet.. every network has a unique MAC address anyway, but biggest is the Internet.)