**Core Concept**
The complement system is a group of proteins that play a crucial role in the immune system by enhancing the ability of antibodies and phagocytic cells to clear microbes. It can be activated through three main pathways: the classical pathway, the alternative pathway, and the lectin pathway. The **complement component C3** is central to all three pathways.

**Why the Correct Answer is Right**
The correct answer is related to the fact that **C3** is the point of convergence for both the classical and alternative pathways of complement activation. Upon activation, **C3** is cleaved into **C3a** and **C3b**, which then participate in various immune responses. This makes **C3** a critical component that can be activated in both common (the point where all pathways converge) as well as alternate pathways.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not represent the specific component that can be activated in both pathways.
**Option B:** Similarly, this option does not accurately identify the complement component that plays this pivotal role.
**Option D:** This choice also fails to correctly identify the component that is central to the activation of both pathways.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **C3** is essential for the activation of the complement system through any of its pathways, making its deficiency particularly immunocompromising.

**Correct Answer:** C. C3