Which IPv6 address is valid?

The answer for this is letter A. A has an ipv6 address that states 2031:0:130F:9C0:876A:130B. The other addresses are not valid because of different reasons. Right off the bat, we can see that D is not valid because of the double: . B is not valid because of the letter G. C is not a valid ipv6 address because this contains the letter H.

It will be wise to familiarize yourself with the different letters that should not be part of the usual ipv6 address. The more that you understand this, the easier it will be for you to eliminate some of the addresses that you have to choose from. Out of the various addresses, only A is valid.