The correct answer is D - all of the above.
Leucocytes are a type of white blood cells that are produced in the bone marrow and most of them circulate continually in the blood stream.
There are two types of leucocytes namely the granules and those without. The one with granules are called granulocytes and they include Neutrophils, eosinophils, and basophils. Those without granules are called agranulocytes and include monocytes and lymphocytes. The lymphocytes are divided into small and large lymphocytes.