Erythropoiesis starts inA. KidneyB. LiverC. SpleenD. Red bone marrow

Erythropoiesis is the process of producing red blood cells. In adults, this process primarily occurs in the red bone marrow, although in fetal development, it can also take place in the liver and spleen.

In adults, erythropoiesis begins in the red bone marrow. The red bone marrow is located in the cavities of certain bones, such as the ribs, vertebrae, and pelvis. This is where hematopoietic stem cells differentiate into red blood cells.

The kidneys are involved in erythropoiesis as well, but they don't start the process. They produce erythropoietin, a hormone that stimulates the red bone marrow to produce red blood cells in response to low oxygen levels in the blood.

Therefore, the correct answer is: D. Red bone marrow