**Core Concept**
Pneumococcus, a type of Gram-positive bacterium, is a leading cause of pneumonia and other invasive infections. The virulence or pathogenicity of pneumococcus is influenced by various factors, including its ability to adhere to and invade host cells, evade the immune system, and produce toxins.

**Why the Correct Answer is Right**
The correct answer is **capsular polysaccharide**, specifically the pneumococcal capsule composed of polysaccharides such as pneumococcal polysaccharide capsule type 3 (Pn3). The capsule plays a crucial role in pneumococcal virulence by preventing phagocytosis and complement-mediated lysis. The capsule also helps pneumococcus to adhere to and invade host cells, such as epithelial cells and macrophages. Furthermore, the capsule is a key virulence factor in pneumococcal meningitis and sepsis.

**Why Each Wrong Option is Incorrect**
**Option A:** Pneumolysin is a toxin produced by pneumococcus that damages host cells and tissues, but it is not the primary factor determining virulence.
**Option B:** Pneumococcal surface proteins, such as pneumococcal surface protein A (PspA), are involved in adherence and invasion, but they are not as critical for virulence as the capsule.
**Option C:** Pneumococcal DNA and RNA are essential for bacterial replication and survival, but they are not the primary factors determining virulence.

**Clinical Pearl / High-Yield Fact**
The pneumococcal capsule is a key target for vaccines, and conjugate vaccines that combine the polysaccharide with a carrier protein have been shown to be highly effective in preventing pneumococcal disease.

**Correct Answer:** C.