**Core Concept**
The complement alternative pathway is a part of the innate immune system that plays a crucial role in defending against pathogens. It can be activated by a variety of substances, including microbial cell walls and host-derived molecules. The alternative pathway is unique in that it can be activated spontaneously, without the need for an antibody.

**Why the Correct Answer is Right**
The correct answer is C3. C3 is a central component of the complement system and is involved in the activation of both the classical and alternative pathways. In the alternative pathway, C3 is cleaved by the alternative pathway C3 convertase, resulting in the formation of C3a and C3b. C3b then binds to the surface of pathogens, marking them for destruction. The alternative pathway C3 convertase is formed by the binding of factor B to C3b, and this complex is stabilized by factor D.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because properdin is involved in stabilizing the alternative pathway C3 convertase, but it is not a major component in the activation of the alternative pathway.
**Option B:** Incorrect because factor B is involved in the alternative pathway, but it is not the major component being tested.
**Option D:** Incorrect because C5 is involved in the terminal pathway of the complement cascade, but it is not directly involved in the activation of the alternative pathway.

**Clinical Pearl / High-Yield Fact**
The alternative pathway can be activated spontaneously, but its activity can be regulated by the presence of certain host-derived molecules, such as factor H and factor I. These molecules can bind to C3b and prevent its deposition on host cells.

**Correct Answer: C. C3.**