Correct Option: A

Most of the WBCs have a life span which is even less than 1 day. White blood cells or leukocytes are cells which form a component of the blood. They are produced in the bone marrow at a rate of about 1000 million per day and help to defend the body against infectious disease and foreign materials as part of the immune system.
There are normally between 4 × 10⁹ and 1.1 × 10¹⁰ white blood cells in a litre of healthy adult blood. In conditions such as leukemia this may rise to as many as 50,000 white blood cells in a single drop of blood (only 1% of blood on average is a white blood cell). As well as in the blood, white cells are also found in large numbers in the lymphatic system, the spleen, and in other body tissues. The white blood cells have a life span of 10 days.