It is a rock not stone in marrow not in bone in bolster not in bed it is not living but not dead?
The answer is the letter R !!
Bone marrow is a red compound found at the centre of large bones in the body. It helps the body to make blood cells and have immunity against diseases. A bone marrow donor has a small operation, where a small hole is made in their hip bone. Some bone marrow is removed, under local anaesthetic. This bone marrow is then given to the bone marrow recipient.