**Core Concept**
The question tests the understanding of streptococcal antigen cross-reactions, particularly with synovial fluid. This involves the concept of molecular mimicry, where antigens from one pathogen share similarities with host tissues or proteins, leading to autoimmune responses. In this case, the focus is on the capsular antigens of Group A streptococci (GAS).

**Why the Correct Answer is Right**
The correct answer, **C. Capsular hyaluronic acid**, is due to the molecular similarity between capsular hyaluronic acid (HA) of GAS and the HA found in synovial fluid. This similarity leads to cross-reactivity, where antibodies produced against GAS capsular HA can also bind to HA in synovial fluid, causing autoimmune reactions like rheumatoid arthritis (RA). This phenomenon is known as molecular mimicry, where the immune system mistakes the host's own tissue for the pathogen.

**Why Each Wrong Option is Incorrect**
**Option A:** Carbohydrate (group A) is a general term and does not specifically refer to the antigen responsible for cross-reactivity with synovial fluid.
**Option B:** Cell wall protein is not directly associated with cross-reactivity with synovial fluid.
**Option D:** Peptidoglycan is a component of the bacterial cell wall, but it is not the specific antigen responsible for the observed cross-reactivity.

**Clinical Pearl / High-Yield Fact**
Molecular mimicry between bacterial antigens and host tissues can lead to autoimmune diseases, highlighting the complex interactions between the host and pathogen. This concept is crucial in understanding the pathogenesis of diseases like rheumatoid arthritis, which is often triggered by streptococcal infections.

**✓ Correct Answer: C. Capsular hyaluronic acid**