Explanation:

Haematopoiesis is the process of formation of blood cells (RBCs, WBCs, and platelets). It primarily occurs in the bone marrow, specifically in the red bone marrow of long bones (like the femur) and flat bones (like the sternum and pelvis).

Why are the other options incorrect?

(A) Lungs → ❌ Incorrect

• Lungs are responsible for gas exchange, not blood cell formation.

(B) Liver → ❌ Incorrect

• In fetuses, haematopoiesis initially occurs in the liver, but in adults, the bone marrow takes over this function.

(C) Pancreas → ❌ Incorrect

• The pancreas is responsible for insulin and digestive enzyme production, not blood cell formation.

Thus, bone marrow is the main site of haematopoiesis in adults.