**Core Concept**
Agglutination is a serological test used to detect the presence of antibodies or antigens. In the context of Salmonella typhi (S. typhi), the 'O' antigen is a somatic antigen that elicits an immune response. The inhibition of agglutination with the 'O' antigen suggests an interaction with a substance that interferes with the antigen-antibody reaction.

**Why the Correct Answer is Right**
The correct answer is a substance that inhibits the agglutination reaction by masking or blocking the 'O' antigen. This is typically achieved through the presence of a specific carbohydrate moiety. The correct answer, **H antigen**, is a lipopolysaccharide component of the S. typhi cell wall that shares a common carbohydrate structure with the 'O' antigen. This structural similarity allows the H antigen to competitively inhibit the agglutination reaction, making it a key factor in serological differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not directly interact with the 'O' antigen.
**Option B:** This option is incorrect because it is not a known inhibitor of the agglutination reaction with the 'O' antigen.
**Option C:** This option is incorrect because it does not possess the necessary carbohydrate moiety to inhibit the agglutination reaction.

**Clinical Pearl / High-Yield Fact**
When performing agglutination tests, it is essential to consider the possibility of antigen-antibody inhibition, particularly when working with substances that share carbohydrate structures with the test antigen.

**Correct Answer: D. H antigen**