Step 1: Understanding the Concept
The WBCs are neutrophils, basophils, eosinophils, lymphocytes and monocytes.
Step 2: Sizes
Monocytes are the largest. Lymphocytes are the smallest WBCs, with a diameter of about 7 to 10 micrometres.
Step 3: Result
So the smallest WBC is the lymphocyte.
Final Answer:
The lymphocyte is the smallest WBC.
\[ \boxed{\text{(D) }\text{lymphocyte}} \]