**Core Concept**
The complement system is a part of the innate immune system that helps to clear pathogens from an organism. It consists of several small proteins found in the blood plasma and on cell surfaces, which work together to eliminate pathogens. The complement system has three main pathways: the classical pathway, the alternative pathway, and the lectin pathway.

**Why the Correct Answer is Right**
The classical and lectin pathways of the complement system converge at the level of C4 and C2, which are components of the classical pathway. C4 is a critical component of the classical pathway and is activated by the binding of C1q to antibody-bound antigens. This activation results in the cleavage of C4 into C4a and C4b, which then binds to C2, leading to the formation of C4b2a, also known as C3 convertase. This C3 convertase is responsible for the cleavage of C3 into C3a and C3b, initiating the amplification of the complement response.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it is not a common complement component for both pathways.
**Option B:** This option is incorrect as it is a component of the alternative pathway, not a common component for both classical and lectin pathways.
**Option C:** This option is incorrect as it is not a correct representation of a complement component.

**Clinical Pearl / High-Yield Fact**
The complement system plays a crucial role in the elimination of pathogens from the body and is an important part of the innate immune response. Understanding the different pathways of the complement system is essential for grasping the broader concepts of immunology and its relevance to various diseases.

**Correct Answer:** C. C4. C4 is a common complement component for both the classical and lectin pathways.