15) E

When the concentration of glucose is high, it gets converted to glycogen and stored in the liver. This is why glucose concentration must be high in the blood vessel connecting stomach&intestines and liver (2). This leaves us with options C and E.

Urea is a waste product produced by the breakdown of protein in the liver. Since it is produced by the liver, it must go back into the blood vessel leaving the liver (3). This leaves us with option E only.