**Core Concept**
The Quellung reaction is a serological test used to diagnose pneumococcal infections. It involves the interaction between the capsular polysaccharide of *Streptococcus pneumoniae* and a specific antibody, leading to visible agglutination under microscopy.

**Why the Correct Answer is Right**
The Quellung reaction is caused by the capsular polysaccharide of *S. pneumoniae* reacting with a specific antibody, known as the Quellung serum, which is raised against the capsular polysaccharide. This interaction leads to the capsular polysaccharide becoming more visible under microscopy, resulting in a characteristic "Quellung" or swelling of the capsule. The Quellung serum is specific to the capsular polysaccharide of *S. pneumoniae* and is used to identify the organism in clinical samples.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the Quellung reaction is not caused by the bacterial cell wall or peptidoglycan layer.
* **Option B:** This option is incorrect because the Quellung reaction is not caused by the teichoic acids present in the bacterial cell wall.
* **Option C:** This option is incorrect because the Quellung reaction is not caused by the virulence factors of *S. pneumoniae*, such as pneumolysin or autolysin.

**Clinical Pearl / High-Yield Fact**
The Quellung reaction is an important diagnostic tool for pneumococcal infections, particularly in cases where the organism is not easily cultivable. It is essential to note that the Quellung reaction is specific to the capsular polysaccharide of *S. pneumoniae* and should not be confused with other serological reactions.

**Correct Answer:** C.