The correct answer is option 3: bone-marrow. The production of red blood cells, also known as erythropoiesis, occurs in the bone marrow. The bone marrow is a soft, spongy tissue found inside the bones, and it plays a crucial role in the production of blood cells. Specifically, the red bone marrow, which is found in the ends of long bones, is responsible for producing red blood cells. These cells are then released into the bloodstream, where they carry oxygen to all parts of the body. Red blood cell production is regulated by a hormone called erythropoietin, which is produced in the kidneys and stimulates the bone marrow to produce more red blood cells. Therefore, option 3, the bone marrow, is the correct answer.