The red blood cells are in the

The correct answer is D. Bone marrow.

Red blood cells are produced in the bone marrow, which is a spongy tissue found inside bones. The bone marrow is also responsible for producing white blood cells and platelets.

The heart is a muscle that pumps blood throughout the body. The liver is an organ that helps to digest food and remove toxins from the blood. Lymph nodes are small, bean-shaped organs that are part of the lymphatic system, which helps to fight infection.

I hope this helps!