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.
