6. Where are red blood cells produced? a. Yellow bone marrow b. Lymphatic vessels c. Heart chambers d. Red bone marrow

The correct answer is d. Red bone marrow.<br /><br />Red blood cells, also known as erythrocytes, are produced in the red bone marrow. The red bone marrow is found in the cavities of certain bones, such as the pelvis, ribs, sternum, and vertebrae. It is responsible for producing red blood cells, which are essential for carrying oxygen throughout the body.<br /><br />The other options are incorrect:<br /><br />a. Yellow bone marrow: Yellow bone marrow primarily stores fat and does not produce red blood cells.<br /><br />b. Lymphatic vessels: Lymphatic vessels are part of the lymphatic system and are involved in the transport of lymph, not the production of red blood cells.<br /><br />c. Heart chambers: The heart chambers are responsible for pumping blood, but they do not produce red blood cells.<br /><br />Therefore, the correct answer is d. Red bone marrow.