**Core Concept:** The complement system is a crucial part of the immune response, consisting of a cascade of proteins that work together to eliminate pathogens, clear immune complexes, and modulate inflammation. Two main pathways - classical and alternative - initiate the complement activation process.

**Why the Correct Answer is Right:** Complement component C3 is involved in both classical and alternative pathways. The activation of these pathways leads to the formation of C3 convertase (C4b2a for classical and C3bBb for alternative), which cleaves C3 into C3a (anaphylatoxin) and C3b (opsonin). C3b then binds to pathogen surfaces, marking them for destruction by phagocytes. C3a is an inflammatory mediator, increasing vascular permeability and attracting phagocytes.

**Why Each Wrong Option is Incorrect:**

A. C4: While C4 is a crucial component in the classical pathway, it is not involved in the alternative pathway.
B. C5: C5 is the final common pathway in both pathways, forming C5a (anaphylatoxin) and C5b, which initiates the terminal pathway.
C. C6-C9: These are the components of the terminal pathway, which leads to the formation of the membrane attack complex (MAC) and cell lysis.
D. C2: C2 is involved in the classical pathway activation, but not the alternative pathway.

**Clinical Pearl:** Complement cascade dysregulation can lead to autoimmune diseases, such as paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS). In these conditions, the deficiency or dysfunction of complement regulatory proteins causes uncontrolled complement activation and damage to host tissues.

**Correct Answer:** C3 (Option D).