**Core Concept**
Opsonization is the process by which a pathogen is marked for ingestion and destruction by phagocytic cells. This process involves the coating of the pathogen with a molecule that enhances its recognition and engulfment by phagocytes. In the context of the complement system, opsonization plays a crucial role in the innate immune response.

**Why the Correct Answer is Right**
Opsonization through the complement system is primarily mediated by the C3b fragment, which is generated during the activation of the classical or alternative pathway of the complement cascade. C3b binds to the surface of pathogens, such as bacteria, and serves as a ligand for the CR1 (complement receptor 1) on the surface of phagocytes. This interaction enables phagocytes to recognize and engulf the opsonized pathogen, thereby facilitating its destruction. The C3b fragment also plays a key role in the formation of the membrane attack complex (MAC), which is responsible for the lysis of pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** C3a is a potent anaphylatoxin that is generated during the activation of the complement cascade, but it is not involved in opsonization. Instead, C3a plays a role in mediating the inflammatory response by promoting vasodilation and increasing vascular permeability.

**Option C:** C5a is another anaphylatoxin that is generated during the activation of the complement cascade. Like C3a, C5a is involved in mediating the inflammatory response, but it is not involved in opsonization.

**Option D:** C5b is a component of the membrane attack complex (MAC), which is responsible for the lysis of pathogens. While C5b plays a critical role in the complement cascade, it is not directly involved in opsonization.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that opsonization through the complement system involves the C3b fragment, which binds to the surface of pathogens and serves as a ligand for CR1 on phagocytes. This interaction is critical for the effective clearance of pathogens by phagocytic cells.

**✓ Correct Answer: B. C3b**