The answer is the last two could work. Let's go through the other answers.
The first one, we're encrypting using a1 the value of X0
concatenating that with encrypting X0 using b0.
This means if the evaluator here is either a1 or b0, they would be able to learn the value of X0.
That would enable the evaluator to decrypt more than one entry in this truth table
if the other entries were encrypted the same way.
The second possibility has a similar problem.
If the evaluator knows a1, they can obtain the value of  X0,
that's not secure because they can decrypt two outputs in this table.
The other two make it so that obtaining X0 depends on both keys.
It depends on knowing both a1 and b0.
An evaluator who knows only one of those can't determine the key for this encryption
since this is the XOR of those two values and it depends on both of them.
They also couldn't do both of these.
They could do the first one if they know a1, but they just get an encrypted value there,
and they couldn't decrypt the second one to get the value of X0--so, either of these can work.
This first one is going to be more efficient because it only involves one encryption.
There might be other ways to combine the keys that would be better and provide more security.
