The right answer is bone marrow (specifically red bone marrow).
Bone marrow is a tissue located in the center of the bones.
It exists in two forms: the yellow marrow (adipose tissue), which takes the name of gray marrow as it ages, and the red marrow, responsible for hematopoiesis. The latter produces the different types of blood cells: white blood cells, red blood cells (erythrocytes, the diminution of their number causes anemia), and platelets.