Erythropoiesis starts in 

• kidney

• liver

• spleen

• red bone marrow

C.

spleen

The process of formation of erythrocytes is erythropoiesis, which is part of haemopoiesis (formation of blood).
The production of blood during different stages is as follows:
During few early weeks of embryonic life-yolk sac.
Some later stages (After 3-4 months) - liver (mainly) along with spleen and lymph nodes.
Later part of gestation and after birth (After 7 months) - red bone marrow  (till 5 yrs)
Bone marrow of long bones (20-25 yrs).
Membranous bones like vertebrae, etc.