Which is a valid shortened IPv6 address for 2001:0db8:0000:0000:0000:8a2e:0000:1337?

You can remove leading 0s in the quartet, and you can condense four zeros to one zero. However, you can use the :: to remove zeros only once. Therefore, 2001:db8:0000::8a2e::1337 and 2001:db8::8a2e::1337 are invalid notations. The address 2001:db8:::8a2e:0000:1337 contains three colons, and therefore, it is an invalid notation.