Haematopoiesis is the process of blood cell formation. It mainly occurs in the bone marrow. The bone marrow is a soft, sponge-like tissue found inside certain bones, such as the femur and the sternum. Here, stem cells differentiate into different types of blood cells, including red blood cells, white blood cells, and platelets.

Option 1: Lungs - The lungs are primarily responsible for the exchange of oxygen and carbon dioxide during respiration. They do not play a role in haematopoiesis.

Option 2: Pancreas - The pancreas is an organ involved in digestion and the regulation of blood sugar levels. It does not have a role in haematopoiesis.

Option 3: Liver - The liver is involved in various metabolic processes and detoxification. It does not produce blood cells.

Option 4: Bone marrow - This is the correct answer. The bone marrow is the primary site of haematopoiesis, where blood cells are produced.

Alert - correct answer should be 4.