Step 1: Understanding the Concept
Macrophages are large phagocytes that engulf microbes, worn-out cells and debris.
Step 2: Find the source
Monocytes are the largest leucocytes. They are agranular and have a kidney-shaped nucleus. After a short stay in blood they move into tissues and become macrophages.
Step 3: Eliminate
Lymphocytes make antibodies, neutrophils are short-lived phagocytes, and basophils release histamine.
Step 4: Conclusion
Monocytes differentiate into macrophages.
Final Answer:
Monocytes differentiate into macrophages.
\[ \boxed{\text{(A) }\text{Monocyte}} \]