Y is relatively easy to figure out. The oxygen concentration greatly increases after blood passes through it. This is obviously the lungs. You can rule out options B, D, E, F.
Now, in organ X, carbon dioxide level increases, salt level decreases, glucose level decreases and oxygen level decreases. The only thing that stands out is the urea level. Urea level in blood only decreases after it has passed the kidneys. So C is the answer.