The correct option is B High urea
When kidneys fail, dialysis is necessary to remove waste products such as urea from the blood. Dialysate, also called dialysis fluid, is a solution of pure water, electrolytes, and salts, such as bicarbonate and sodium. So the dialysing unit contains a fluid that is the same as plasma except that it has high urea, which will be removed during this process. The way this works is through a process called diffusion.