**Core Concept**
Lancefield grouping is a method used to classify streptococci based on the presence or absence of specific carbohydrate antigens on their cell walls. This classification system helps in identifying the group A streptococci, which are commonly responsible for streptococcal infections.

**Why the Correct Answer is Right**
The Lancefield grouping is achieved by using antisera that react with specific carbohydrate antigens present on the cell walls of streptococci. The antisera are prepared in rabbits and are specific to each group. The reaction between the antisera and the cell wall antigens is detected using precipitation techniques. The group A streptococci have the C-carbohydrate antigen, which reacts with the group A antisera. This reaction is used to identify the group A streptococci. The Lancefield grouping is an important tool in the diagnosis and epidemiology of streptococcal infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Lancefield grouping is not done using bacterial toxins.
**Option B:** This option is incorrect because the Lancefield grouping is not done using agglutination techniques.
**Option C:** This option is incorrect because the Lancefield grouping is not done using biochemical tests.

**Clinical Pearl / High-Yield Fact**
The Lancefield grouping is an important tool in the diagnosis of streptococcal infections, especially in the identification of group A streptococci, which are commonly responsible for infections such as strep throat and rheumatic fever.

**Correct Answer: D. Antisera. Lancefield grouping is done by using antisera that react with specific carbohydrate antigens on the cell walls of streptococci.**