Final answer:
The primary cause of anemia in patients with renal failure is Erythropoietin deficiency (C).
Step-by-step explanation:
The primary cause of anemia in patients with renal failure is Erythropoietin deficiency (C). Erythropoietin (EPO) is a hormone produced by the kidneys that stimulates the production of red blood cells in the bone marrow. When the kidneys are not functioning properly, EPO production is reduced, leading to a decrease in red blood cell production and subsequently causing anemia.