The answer is the second choice. The first choice doesn't make any sense at all.
The difference between the second choice and the third choice is whether we use
the encryption function or decryption.
The reason that we're using encryption function just like we did in encrypting
is because when we're decrypting we have these cipher text,
and we're trying to get the messages, so we're just trying to go through these XORs
in the reverse direction.
That means we need the same exact thing that we had in encryption to XOR out,
which is the result of encrypting these Xi values.
