**Core Concept**
The Lancefield grouping is a method used to classify beta-hemolytic streptococci based on the presence of specific C-carbohydrate antigens on their cell walls, which are responsible for their group-specific serological reactions.

**Why the Correct Answer is Right**
The Lancefield grouping is performed by using antisera that contain antibodies against the C-carbohydrate antigens. When a beta-hemolytic streptococcus is mixed with the corresponding antisera, it will agglutinate (clump together) if the antibodies bind to the antigens present on the bacterial cell wall. This reaction is specific to each group of streptococci, allowing for their identification and classification. The antibodies in the antisera react with the surface antigens of the streptococci, resulting in visible clumping, which is indicative of a positive reaction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the Lancefield grouping method.
**Option B:** This option is incorrect because the Lancefield grouping is not based on colony morphology or biochemical reactions.
**Option C:** This option is incorrect because the Lancefield grouping is not performed using a specific antibiotic or antimicrobial agent.

**Clinical Pearl / High-Yield Fact**
The Lancefield grouping is an important tool for differentiating between various groups of beta-hemolytic streptococci, which can aid in the diagnosis and treatment of streptococcal infections, such as strep throat or skin infections.

**Correct Answer: C. Phosphoric acid.**