Erythropoiesis starts in

Erythropoiesis (production of RBC) occurs in the red bone marrow in the adults. Erythrocyte development starts from common myeloid progenitor cells (secondary stem cells) under the influence of the hormone erythropoietin produce by the kidneys.

Haemopoiesis occurs sequentially in three major organs:

1. The yolk sac in the early developmental stages of the embryo
2. The liver during the second trimester of pregnancy
3. The bone marrow during the third trimester

The spleen participates to a very limited degree during the second trimester of pregnancy.

At birth, most haemopoiesis occurs in the red bone marrow, as it occurs in the adult.

Note: The question is ambiguous. There would be no ambiguity if the question was “In adult humans, erythropoiesis occurs/starts in”. As the question reads “Erythropoiesis starts in”, the perfect answer would be yolk sac, which is not available among the options. Most sources give “red bone marrow” as the answer, through some prefer “liver”, which also is not the perfect answer.