Chief components of the immune system are

The correct option is A Leucocytes
Leucocytes or the white blood cells are the disease-fighting white blood cells that identify and eliminate pathogens. Hence, they are one of the chief components of the innate immune system. A high white blood cell count is referred to as leucocytosis. The innate leucocytes include phagocytes (macrophages, neutrophils and dendritic cells), mast cells, eosinophils and basophils.

Therefore, the correct answer is option C.