The correct option is D kidney
Erythropoietin (EPO), is a glycoprotein cytokine secreted by the kidney. It helps in the formation of red blood cells by the bone marrow. The kidney cells that make EPO are specialized and are sensitive to low oxygen levels in the blood coming into the kidney. After being released into the blood stream it binds with receptors in the bone marrow, where it stimulates the production of red blood cells (erythrocytes). Medically, EPO is used to treat certain forms of anaemia.