A direct indication that the white blood cells of the body are functioning would be

A. an increase in the number of oxygen molecules in the lungs

B. a decrease in the number of pathogens in the body

C. a decreased secretion of hormones by certain glands

D. an increase of carbon dioxide in the cells of the body

B. a decrease in the number of pathogens in the body

Explanation:

White blood cell includes immune cell like monocytes, macrophages, neutrophils, basophils, eosinophils, etc. These cells are responsible for fighting with the pathogens or foreign molecules and eliminate them from the body.

So when any pathogens get enter in the body these cell kills those pathogen and protect us from them. Therefore the decrease in the number of pathogens in the body is a direct indication that the white blood cells in the body are working. So the right answer is B.