<prompt> Opsonin among the following?C3aC3bC4aC5a  <response>: Opsonization and phagocytosis: C3b and its cleavage product iC3b inactive C3b, when fixed to a microbialcell wall, act as opsonins and promote phagocytosis by neutrophils and macrophages, which bear cell surface receptors for the complement fragments.Ref: Robbins and Cotran Pathologic Basis of Disease; 9th edition; Chapter 3; Inflammation and Repair; Page no:89 