Final answer:
C3a functions as a chemoattractant to attract phagocytes, while C3b serves as an opsonin to tag pathogens for phagocytosis by phagocytes. The correct answer to the question is a. Is a chemoattractant; Is an opsonin.
Step-by-step explanation:
When C3 is cleaved into two fragments, C3a and C3b, each serves different functions within the immune response. C3a acts as a chemoattractant which diffuses away from the site of activation and attracts phagocytes to the site of infection.
On the other hand, C3b binds to the surface of the pathogen, marking it for phagocytosis, functioning as an opsonin.
Once pathogens are opsonized with C3b, phagocytic cells such as macrophages and neutrophils can more easily bind to and engulf them.
Given the functions of C3a as a chemoattractant and C3b as an opsonin, the correct answer to the question is: C3a is a chemoattractant; C3b is an opsonin, which corresponds to option a.