Final answer:
The larger fragment, C3b, binds to the surface of the pathogen and activates the rest of the cascade to form the membrane-attack complex (MAC).
Step-by-step explanation:
In the complement system, when the C3 protein is split, it forms C3b and C3a fragments. The larger fragment, C3b, binds to the surface of the pathogen, while the smaller fragment, C3a, attracts phagocytes to the site of infection. Surface-bound C3b then activates the rest of the cascade, leading to the formation of the membrane-attack complex (MAC), which can kill certain pathogens by disrupting their osmotic balance. Therefore, the answer to your question is A. Membrane Attack Complex (MAC).