**Core Concept**
Opsonins are molecules that mark pathogens for destruction by the immune system, facilitating phagocytosis. In this context, the correct answer refers to a specific type of opsonin that plays a crucial role in the innate immune response.

**Why the Correct Answer is Right**
Complement component C3b is an opsonin that binds to pathogens, making them more susceptible to phagocytosis by neutrophils and macrophages. This process involves the activation of the classical or alternative complement pathway, leading to the cleavage of C3 into C3a and C3b. C3b then covalently binds to the pathogen's surface, marking it for destruction. The recognition of C3b by complement receptor 1 (CR1) on phagocytic cells triggers the engulfment and degradation of the opsonized pathogen.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is an antibody that can act as an opsonin, but it is not the correct answer in this context. IgG binds to pathogens through its Fc region, but it is not a complement component.
**Option B:** IgM is a pentameric antibody that plays a key role in the classical complement pathway, but it is not an opsonin itself. IgM can activate the classical pathway, leading to the formation of C3b, but it is not the correct answer.
**Option C:** Lysozyme is an enzyme that breaks down bacterial cell walls, but it is not an opsonin. Lysozyme can inhibit the growth of certain bacteria, but it does not mark pathogens for destruction.

**Clinical Pearl / High-Yield Fact**
Complement component C3b is a key opsonin that plays a crucial role in the innate immune response. Its ability to mark pathogens for destruction is essential for the clearance of infections.

**Correct Answer:** C. Complement component C3b.