**Core Concept**
Adhesion of microorganisms to host cells is a crucial step in the pathogenesis of various infections. This process involves the interaction between microbial surface structures and host cell receptors, leading to the formation of a biofilm or a stable attachment.

**Why the Correct Answer is Right**
The primary mechanism of adhesion involves the expression of microbial surface proteins or glycoproteins, known as adhesins, which bind to specific host cell receptors. For example, the FimH adhesin on E. coli binds to mannose residues on the surface of host cells. This interaction triggers a series of signaling events that facilitate the internalization of the microorganism, leading to infection. The host cell receptors involved in adhesion include integrins, selectins, and glycosaminoglycans.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the mechanism of adhesion.
**Option B:** This option is incorrect because it is too broad and does not accurately describe the process of adhesion.
**Option C:** This option is incorrect because it is a type of microbial structure, but it is not directly involved in the process of adhesion.

**Clinical Pearl / High-Yield Fact**
The ability of microorganisms to adhere to host cells is a key factor in their pathogenicity, and understanding the mechanisms of adhesion is crucial for the development of effective antimicrobial therapies.

**Correct Answer:** C. Fimbriae. Fimbriae are hair-like structures on the surface of certain bacteria that play a key role in their adhesion to host cells.