**Core Concept**
The Ouchterlony procedure is a type of immunodiffusion technique used to detect and analyze the interaction between antibodies and antigens. This method relies on the principle of diffusion of proteins in a gel matrix, allowing for the visualization of immune complex formation.

**Why the Correct Answer is Right**
In the Ouchterlony procedure, two or more proteins (antibodies or antigens) are placed in separate wells within a gel matrix, and the resulting diffusion of proteins creates a visible precipitin line where the proteins interact. This interaction is a result of the formation of immune complexes between the antibodies and antigens, which can be visualized as a precipitate. The Ouchterlony procedure is particularly useful for identifying and characterizing the specificity of antibodies and antigens.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the type of immunodiffusion technique being used in the Ouchterlony procedure.
**Option B:** This option is incorrect because it does not accurately describe the principle behind the Ouchterlony procedure.
**Option D:** This option is incorrect because it does not specify the type of immunodiffusion technique being used in the Ouchterlony procedure.

**Clinical Pearl / High-Yield Fact**
The Ouchterlony procedure is a widely used technique in immunology for detecting and characterizing antibodies and antigens, and is often used in the diagnosis of infectious diseases and autoimmune disorders.

**Correct Answer: A. Agar gel.**