This is rather a simple question.
The two large blood vessels are the vena cava and the aorta. We can distinguish between 1 and 2 by their lumen size. The blood vessel with the larger lumen is the vena cava and the one with the smaller lumen is the aorta.
Therefore, 1 is the vena cava and 2 is the aorta.
The blood vessel coming out of the aorta (4) is the renal artery and the one coming out of the vena cava (3) is the renal vein.
Since most of the the urea is deposited in the kidney, the blood coming out of the kidney would have little to no urea. Hence, the renal vein would have the least concentration of urea.
All of the urea which is deposited into the kidneys then travels to the ureter (5) which connects the kidneys to the bladder. Therefore, the highest concentration of urea would be in the ureter (5).