There are two types of bone marrow:

* Red bone marrow: This is the type of bone marrow responsible for producing blood cells. It contains hematopoietic stem cells, which are capable of differentiating into all types of blood cells, including red blood cells, white blood cells, and platelets.

* Yellow bone marrow: This type of bone marrow is mainly composed of fat cells and does not produce blood cells. However, it can revert to red bone marrow if the body needs more blood cells.

So, the answer to your question is bone marrow, specifically red bone marrow.