**Core Concept**
The alternate complement pathway, also known as the properdin pathway, is one of the three pathways of the complement system that helps to clear pathogens from an organism. This pathway is initiated without the presence of antibodies and is crucial for the innate immune response.

**Why the Correct Answer is Right**
The correct answer is C3 convertase in the alternate complement pathway, which is a complex of factor B and factor D. This complex cleaves C3 into C3a and C3b, leading to the amplification of the complement response. The alternate pathway is initiated by the recognition of pathogens by factor B, which is then cleaved by factor D to form the C3 convertase. This convertase then cleaves C3 to produce C3a and C3b, which in turn activate the complement response.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the alternate complement pathway, and factor B and factor D are not the correct C3 convertase in the classical pathway.
**Option B:** This option is incorrect because C3 convertase in the classical pathway is a complex of C4b and C2b, not factor B and factor D.
**Option C:** This option is incorrect because it does not specify the correct components of the C3 convertase in the alternate complement pathway.

**Clinical Pearl / High-Yield Fact**
The alternate complement pathway is a key component of the innate immune response and plays a crucial role in the clearance of pathogens from the body. Understanding the different pathways of the complement system is essential for understanding the immune response and the pathogenesis of various diseases.

**Correct Answer: C. C3 convertase in the alternate complement pathway is a complex of factor B and factor D.**