The correct answer is regulation of lymphocyte production.
Kidneys have a function in:
· regulation of blood pressure (through the regulation of the volume of blood in the body);
· regulation of acid-base balance (by excreting H+ into the urine and reabsorbing HCO3- from the urine);
· removal of wastes from the blood (through the urine);
· regulation of erythrocyte production (producing erythropoietin hormone).