The answer is the 4th choice.
If she can find 2 keys where either the encryption of 0 with key 0
is equal to the encryption of 1 with key 1 or the opposite,
then at least half the time she has an advantage.
So in this case, if she sends this as the message, if Bob guesses 0, she can win
by revealing K1.
If Bob guesses 1, she can win by revealing K0, so she's guaranteed to win.
If she finds a key pair like this, well, then she can do the same thing.
She can reveal this is the message and then based on Bob's guess,
reveal wither K1 or K0.
Either of these would also allow her to cheat if she could find a key that has this property--
that for whatever key she happened to pick, encrypt 0 to the same thing that key encrypted 1 to,
well, that would allow her to cheat just as well.
But this is a harder thing to do than finding 2 keys that have this property.
We might not know enough yet to be able to be sure that this is harder than that,
but we know it certainly is no more difficult and this could be easier,
and we'll see that, in fact, it is easier.
The second one would also be enough, but this is the much stronger property
to find a key pair that has both of those properties.
How hard this is depends on lots of things.
The first thing we have to answer a little more is what encryption means here.
