The function of erythropoietin is a. reabsorption of sodium ions. b. excretion of potassium ions. c. reabsorption of water. d. to stimulate red blood cell production. e. to increase blood pressure.

Erythropoietin is a hormone produced primarily by the kidneys. It plays a crucial role in the production of red blood cells (erythropoiesis) in the bone marrow. When the body senses a decrease in oxygen levels, it increases the production of erythropoietin to stimulate the production of more red blood cells, which carry oxygen to various tissues and organs.

Now that we understand the role of erythropoietin let's evaluate each option: a. reabsorption of sodium ions: This is related to the function of the kidney in maintaining electrolyte balance but not directly related to erythropoietin. b. excretion of potassium ions: Similar to option a, this is related to the kidney's function but not to erythropoietin. c. reabsorption of water: This process takes place in the kidney as well but isn't related to erythropoietin. This is mainly regulated by antidiuretic hormone (ADH). d. to stimulate red blood cell production: This option correctly describes the function of erythropoietin, as discussed earlier. e. to increase blood pressure: While increased red blood cell production can ultimately lead to increased blood pressure, it is not the primary function of erythropoietin. The primary function is to stimulate red blood cell production.

Based on the evaluation of the given options, the correct answer is: d. to stimulate red blood cell production.