The correct answer is: A. As a defence against infection.
White blood cells, also known as leukocytes, are a type of blood cell that plays a major role in the body’s immune system. They are produced in the bone marrow and circulate in the blood and lymph. White blood cells are responsible for fighting infection and disease. They do this by engulfing and destroying bacteria, viruses, and other foreign invaders. White blood cells also produce antibodies, which are proteins that help the body recognize and destroy harmful substances.
Option B is incorrect because white blood cells are not a source of energy. Energy is provided by carbohydrates, fats, and proteins.
Option C is incorrect because white blood cells do not clot blood. Blood clotting is a process that is controlled by platelets and coagulation factors.
Option D is incorrect because white blood cells do not transport oxygen. Oxygen is transported by hemoglobin, a protein found in red blood cells.