The correct option is D Leukemia
Leukocytosis is the condition of abnormal increase in the WBC count. It happens during:
∙ infection so as to combat it.
∙ inflammation due to chemical irritants and auto-immune diseases.
∙ allergy due to elevated immune response.
∙ leukemia as there is an explosive proliferation of WBCs. In this case, the people are highly susceptible to infection as these WBCs have not matured.