The best answer is that it does not provide any authentication.
It does establish a shared key,
but it's a shared key with some unknown entity.
It's may be really providing no benefit,
but I would say that at least establishing the shared key means
now you've got a channel with some entity that's encrypted,
and if someone is eavesdropping on that channel--
you're only worried about a passive adversary--
they might be able to eavesdrop on that channel, but you do have a secure key established.
You don't know that it's with the right server.
If you there's an active adversary, this definitely provides no benefit,
because the active adversary can do the attack in the middle,
impersonate the server, establish the key,
and that's the one who has the shared key instead of the server that you intend to talk to.
If it's a passive adversary, that assumes the message actually does get to the right place,
so it provides some security against the passive adversary,
who can't intercept or inject new messages.
It allows you to establish that key, but doesn't provide any authentication.
I think the 3rd answer is the best.
One could make arguments for either the 2nd or the 4th.
Don't be upset if you had good arguments for those,
and I should emphasize that this establishes a secured shared key
only against a passive adversary even without worrying about the authentication
since an active adversary can interfere with the key that is established.
For this protocol to provide a meaningful benefit,
it's necessary that the client actually knows this KUs value or has a way to check it
before accepting that this is the server that she intends to talk with.
