Answer:
The answer to the question "Which of the following 'flags' a pathogen for destruction by the phagocytic cell?" is "c. C3b."
Step-by-step explanation:
As mentioned in the search results [1], [3], and [5], C3b is an opsonin produced from the cleavage of C3 by C3-convertase. Opsonins, such as C3b, act as molecular flags, signaling the presence of pathogens or other foreign cells to immune cells, such as phagocytes, so that they can be engulfed and destroyed. Therefore, C3b flags a pathogen for destruction by a phagocytic cell.